How many students are there in the classroom— . They are all in the lab.

下列各句中，没有语病的一项是（ ）

The researchers found clear and convincing evidence _______ happiness is the key when it comes to better health and longer lives.

Most of the young people enjoy ______ Jay Chou’s songs. A．sing

23．Can you make a sentence to _____ the meaning of the phrase?