DISCUSSION: This question gives no hints. This is the strategy you should use on questions with no hints:
- Don’t spend time on any of the answers until you’ve looked at all of them.
- Your goal should be to identify the 1-2 most plausible answers, quickly.
- Then, check the passage to see if you can confirm them.
The wrong answers here are designed to bog you down. Don’t fall into their trap. Choose the order you approach them in.
- Not so. CFCs were the main refrigerant chemicals. But that doesn’t mean no others were discovered. Maybe others existed, but were more expensive.
- CORRECT. Lines 55-60 support this. “Ozone depleting gases” were banned, and “environmentally friendly” substitutes were developed. Taken together, these two terms suggests that the new refrigerants were environmentally friendly because they didn’t deplete ozone.
(Technically, these substitutes were developed “after the late 1970s”, but it’s reasonable to assume industry didn’t suddenly switch back to CFCs after 1987. This is a “most strongly supported” question)
- We don’t know why CFCs were used. they simply might have been the cheapest or most readily available chemicals. (That doesn’t necessarily mean energy efficient. E.g. an alternative might be 2% more efficient and 1000x more expensive.)
- Lines 55-56 say that CFCs were banned in North America in the 1970s. That’s before the Montreal protocol.
- We don’t know anything about this. The only information about new refrigerant chemicals is in lines 56-60, which says they’re more environmentally friendly. That could mean they’re 100% environmentally friendly!