According to surveys by the National Institute on Drug Abuse, about 20 percent of young adults used cocaine in 1979, doubling those reported in the 1977 survey.
A. doubling those reported in the 1977 survey
B. to double the number the 1977 survey reported
C. twice those the 1977 survey reported
D. twice as much as those reported in the 1977 survey
E. twice the number reported in the 1977 survey
[spoiler]OA: E[/spoiler]

I posted this question to understand the usage of X%Y

1. X%Y and y is plural and countable entity (E.g. 20% of the apples were/was sold)
would X%Y be [plural and countable entity] or [Non countable entity] ?
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2. X%Y and y is Non- countable entity (E.g. 20% of the oil was sold)
would X%Y be singular Non- countable entity

In Option D:twice as much as those reported in the 1977 survey
those here is plural .
Want to know if '20% of young adults is plural or singular, countable or non countable' --