A ship in harbor is safe, but that’s not ______ ships are built for.
| A．what | B．whom | C．why | D．when |
We couldn’t understand: what could ______ his sudden change of mind?
| A．enjoy | B．explain | C．accept | D．mean |
I didn’t mean to break your vase—it was just ______.
| A．an accident | B．an advantage | C．an agreement | D．an experience |
He was breathing ______ after the relay race.
| A．heavily | B．slowly | C．carefully | D．weakly |
People are concerned about the quality of the air they ______.
| A．hear | B．eat | C．drink | D．breathe |
She walked home by herself, ______ she knew that it was dangerous.
| A．if | B．so that | C．although | D．because |