Last question please help thanks

There are three of chromosome number 18

Explanation: normally every chromosome is a pair, this is a triplet which is a genetic disorter

the answer to your question is slate

i'm thinking (a) to keep sand in place; ////////////

explanation: a sand fence or sandbreak, like a snow fence, is used to force windblown, drifting sand to accumulate in a desired place. sand fences are employed to control erosion, sand dune stabilization, keep sand off roadways, and to recruit new material in desert areas.