Do you know where ... Mississippi River is? a) an b) the c) - He ... twenty in a month a) will be b) am going to be c) is We ... go to Peru next year. We`re not sure. a) might b) should c) probably That`s the woman ... son is an actor. a) who b) whose c) which This afternoon, we ... sandboarding. a) are going b) are going c)will I`m bored ... camping trips. Let`s go on a safari. a) from b) of c) for Перед вами такое упражнение, похожее на викторину. Ответы я знаю, поэтому в этом задание не заключается. Тут вы должны проявить свои знания и написать, объяснить мне почему тут именно этот вариант ответа. Это ОЧЕНЬ НУЖНО, и ОБЪЯСНЯЙТЕ, пожалуйста, ПОНЯТНО. Заранее всем спасибо!

Do you know where the Mississippi River is?
THE is the definite article.   when you have ONE of something that is definite, you use THE.  there is only one mississippi river, so you must use the.

an is an indefinite article, not appropriate. if you have one of something thats not specific such as an apple. 

He will be twenty in a month.
in a month says this is in the future, so you need a future tense verb
am goes with I always  (I am ...)
is is present tense, but you could say he is going to be twenty in a month

We might go to Peru next year.  Were not sure.
not sure means it may not happen. might is also uncertain
so should means we need to do something

thats the woman whose son is an actor.
i am not sure why, just that this one is correct

This afternoon, we are going sandboarding
a) and b) are the same answer
this afternoon means future tense
going to states a plan or intention, where will is used in a different sense, as in a promise or request for help

im bored of camping trips.  lets go on a safari
for is definitely wrong
i have heard from used, but i suspect it is not proper grammar

извините за все английский, мой руский не достаточно хорошо.