Question

1. Scientists think that wild animals must___ the blance of ecosystem.A. keep B. To keep C. Be keeping D. Be kept 2. Pollution can have serious effects___ the balance of ecosytem. A. In B. On C. To D. For

Answer 1

a (must là động từ khiếm khuyết nên dùng động từ nguyên mẫu sau nó)

b (effects + on: ảnh hưởng)

Answer 2

1. Scientists think that wild animals must___ the blance of ecosystem.

A. keep B. To keep C. Be keeping D. Be kept 2

. Pollution can have serious effects___ the balance of ecosytem.

A. In B. On C. To D. For