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Ecologists are trying to preserve our environment for future generations by protecting the ozone layer, purifying the air, and have planted the trees that have been cut down.

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Ecologists are trying to preserve our environment for future generations by protecting the ozone layer, purifying the air, and have planted the trees that have been cut down.

to preserve

for

the ozone layer

have planted

Giải thích

Chọn đáp án Dhave planted => planting
Giải thích: các động từ theo sau “by” được nối với nhau bởi “and” nên được chia V-ing giống nhau (protecting, purifying)