Less or few
When drive-ins were at the height of their popularity in the late 1950’s, some 4,000 existed in the United States, but today there are less than one-quarter that many.
a. there are less than one-quarter that many
b. there are fewer than one-quarter as many
c. there are fewer than one-quarter of that amount
d. the number is less than one-quarter the amount
e. it is less than one-quarter of that amount
Please comment on how to figure out such questions
C seems to be the correct answer