he teacher told them since light ________ (travel) faster than sound,lightning ________ (appear) to go before thunder.
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《煤矿安全规程》规定井下粉尘浓度每月必须至少测2次()
The director recommended that she ________ (study) more English before going abroad.
How long ________ they ________ (dig) the ditch?
Night ________ (fall), we hurried home.