The stock market crash of 2020 has been painful, for sure. But for those with a long-term view and looking to invest for decades, it’s thrown up a lot of FTSE 100 opportunities. Investing in a Stocks and Shares ISA when share prices are down could be your best plan for a comfortable retirement.We still need to be careful, and not just buy anything with a fallen share price. No, a lemon is still a lemon, no matter what its price, and I strongly recommend a lemon-free ISA. With that in mind, I’m looking at two companies releasing updates Friday. I’d buy one, but I wouldn’t touch the other with a barge pole.5G is here – and shares of this ‘sleeping giant’ could be a great way for you to potentially profit!According to one leading industry firm, the 5G boom could create a global industry worth US$12.3 TRILLION out of thin air…And if you click here we’ll show you something that could be key to unlocking 5G’s full potential…Barge pole stockThe FTSE 100 stock I’m not going near is Pearson (LSE: PSON). The educational publisher gave us a first-half report, and the Covid-19 slump makes it tricky to evaluate. Underlying revenue fell 17% on the prior year, though the company puts that down largely to the pandemic.Pearson also suffered an adjusted operating loss of £23m, though in the circumstances that’s probably not too bad. The cash situation looks comfortable enough, so I’m not concerned for the firm’s survival. Net debt stood at £982m at 30 June, but the company’s available liquidity was put at £1.6bn. The interim dividend was held at 6p, the same as last year. Liquidity is a key factor in my ISA decisions, but on its own it’s not enough.Pearson has already been struggling with the shift away from printed materials and to online teaching aids. The competition in the virtual space is more intense, has lower barriers to entry, and prices need to be ever lower. Add to that the devastation caused to the US educational market by the lockdown, and I have serious concerns.The Pearson share price is down more than 50% over five years, but it’s still not an ISA candidate for me. I see more pain before there’s any gain to be had.ISA buyI’m seeing far more that’s attractive in Ferguson (LSE: FERG), whose share price has gained in 2020. It’s not up much, at 2.4%, but anything positive in this Covid-19 year suggests a long-term winner.Ferguson is the world’s largest heating and plumbing distributor, and I see that as a very defensive business to be in. Even with lockdown headwinds, it’s an essential business sector that should continue to do well. And I reckon every Stocks and Shares ISA should be built on a bedrock of defensive shares.According to Friday’s update, trading has been consistently improving from the lockdown low point. April was tough with revenue from continuing operations down 15.3% that month, year-on-year. But the period from 1 May to 21 July saw it pull back to a modest 3.6% drop.On the liquidity front, the firm estimates its net debt to adjusted EBITDA ratio at less than one, which is very healthy. I’d love an ISA full of stocks in that happy position. Ferguson is a firm ISA buy for me. Alan Oscroft | Friday, 24th July, 2020 | More on: FERG PSON Image source: Getty Images Simply click below to discover how you can take advantage of this. 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A FTSE 100 stock I’d buy for my ISA today, and one I’d steer clear of I’m sure you’ll agree that’s quite the statement from Motley Fool Co-Founder Tom Gardner.But since our US analyst team first recommended shares in this unique tech stock back in 2016, the value has soared.What’s more, we firmly believe there’s still plenty of upside in its future. In fact, even throughout the current coronavirus crisis, its performance has been beating Wall St expectations.And right now, we’re giving you a chance to discover exactly what has got our analysts all fired up about this niche industry phenomenon, in our FREE special report, A Top US Share From The Motley Fool. Alan Oscroft has no position in any of the shares mentioned. The Motley Fool UK has recommended Pearson. Views expressed on the companies mentioned in this article are those of the writer and therefore may differ from the official recommendations we make in our subscription services such as Share Advisor, Hidden Winners and Pro. 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