医生到达时，他的心脏已经停止了跳动。

By the time the doctor had arrived, his heart had stopped ___beating.

我的心像铃铛一样跳动，我非常兴奋。

My heart was ___beating like a bell and I was really excited.

当我紧张时，我的心跳得很快。

When I'm nervous, my heart ___beats quickly.