30.Humans, _____________, interact through communicative behavior by means of signs or symbols used conventionally.
(A) like other animals (B) how other animals (C) other animals that (D) do other animals
10. According to recent scientific studies, _____ from cars is the main cause of global warming.
(A) pollution (B) solution (C) selection (D) population
21. I ________ for more than 10 years, and now I am still taking English classes at school.
(A) study English
(B) studied English
(C) am studying English
(D) have been studying English