Which human activity has the most negative effect on the stability of a river enviorment

Littering

Explanation:

100 people throw 100 plastic bags in the water. Big mistake. AND THAT'S JUST 100 PEOPLE! Imagine thousands of people every year and 300 of the litter, that really can destroy the enviroment. But let's say 10,000 people go to the river and 2000 litter. Now you've really messed up.

of those who start drinking before they're 15 will develop alcohol dependence

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 : )