Nitrous acid plays an important role in chemical reactions.

He played a serenade for her.

He caught a fish in the deep sea.

We should reduce our energy consumption.

He racked his brains all night to solve the exam questions.

The crescent moon is beautiful, isn't it?

He is good at playing the xiao flute.

He is receiving treatment for obsessive-compulsive disorder.

My grandfather underwent cataract surgery.

He found a ceremonial libation cup used in ancient ceremonies.