The highest rates of illicit drug use occur among adults aged

18-20 years

Explanation:

The highest rates of illicit drug use occur among adults aged __18-20 years__.

A recent study was conducted by scientist in which they observed highest illicit drug use among teenage adults. Here is the reference to that paper:

Egan, K. L., Suerken, C., Debinski, B., Reboussin, B. A., Wagoner, K. G., Sutfin, E. L., & Wolfson, M. (2019). More than just Alcohol: Marijuana and Illicit Drug Use at Parties Attended by 15–20 Year Olds. Substance use & misuse, 54(2), 297-306.

the answer would be 2, if crossing over doesn’t occur 2 out of 4 of the daughter cells will have that allele.

hope this : )