a phenomenon that(, most scientists agree,) is caused by human beings's burning of fossil fuels<--非限制性的插入, 所以author是認為/a fact這個phenomenon是caused by human beings's burning of fossil fuels, 而不是most scientists認為 a phenomenon that most scientists agree is caused by human beings's burning of fossil fuels<--限制性的插入, 所以是most scientists認為這個phenomenon是caused by human beings's burning of fossil fuels
有時候, 插入的東東很短, 有些書建議把commas omitted.
Anyway, 這題不是考comma, 而這種comma omission (if I remember correctly) 從來不是考點.
OG50.in the economy to avoid the recession many were fearing earlier in the year, and rather to come 这里OG的解释是the recession must be followed by that. 这句recession后面如果加了that,后面的是同位语从句,而不是定语从句。所以一定要加that。