托福词汇猜测题之maintain

But in many cases consumers are misled if they believe organic foods can maintain health and provide better nutritional quality than conventionally grown foods. So there is real cause for concern if consumers, particularly those with limited incomes, distrust the regular food supply and buy only expensive organic foods instead.

　　The word "maintain" is closest in meaning to which of following?

　　(A) improve

　　(B) monitor

　　(C) preserve

　　(D) restore