Trees are useful to man mainly in three ways, the most important of which is that they can protect ______ from droughts (干旱) and floods.
“Things________never come again!” I couldn’t help talking to myself.
Every one of us must know:________we live can we save the earth.
—Can those________at the back of the classroom hear me? —No problem.
Attracting _____ senior citizens, Florida is _____ home to the largest population of elder Americans.
—Bob feels really sorry, Jean. — ______ I wouldn’t accept his excuses unless he takes back what he said.