Researchers have found clear and convincing evidence ________ happiness isthe key to better health and longer lives

These problems, ________, will seriously affect the growth of teenagers and even the nation’s future.

下列句子中，没有语病的一项是（ ）

Could I have a word with you, mum?— Oh dear, if you __________.

Let’s __________the subway to the park.- That _________ good.