A．whenB．whatC．whyD．how

（)they are doing has never been done before.A. ThatB. WhatC. WhyD. Whether

【C16】A．thatB．whatC．whyD．how

【C13】A．whenB．whatC．thatD．if

【C15】A．howB．whatC．whyD．which

【C9】A．whenB．whereC．whyD．how

【C17】A．whenB．whereC．whyD．how