Using bicycles ______ greatly to people’s physical fitness as well as easing traffic jams.

If he ______ my advice, he wouldn’t have lost his job.

下列各句中，加点的成语使用正确的一项是（ ）

I like __________in the early morning when the air is fresh and I can take a walk around the garden.

The two men talked in a low voice, ____nobody heard what they were saying