Chia dạng đúng của từ trong ngoặc

1) He is writing a letter of ( apply) 
2) He applied to become an American ( citizenship)
3) As a child he received most of his ( educate) at home 
4) People are becoming far more aware of ( environment ) issues
5) I ( enroll ) for the course yesterday 
6) Her ( explain ) was unclear 
7) Football takes place ( nation )
8) WHO is an international ( organize)
9) The price of petrol fell ( slight ) last month 
10) We must preserve ( nature) resources