If there are ______ trees, the air in our city will be _______ cleaner.

下列各句中，没有语病的一项是（ ）

Miss White has decided to explore in the forest by herself.—She________ask her boyfriend to if she ________go walking in the forest.

Shelly had prepared carefully for her biologyexamination so that she could be sure of passing it on her first_________.

Our teacher was very happy because __________failed the examination.