800 câu trắc nghiệm Điền từ Tiếng Anh có đáp án - Phần 5

Conservation activities for nonrenewable resources focus (2) _______ maintaining an adequate supply of these resources well into the future.

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Conservation activities for nonrenewable resources focus (2) _______ maintaining an adequate supply of these resources well into the future.

in

on

at

by

Giải thích

B là đáp án đúng