When you are using gas, ________ that one of the windows is open.

A.be sure

B.make sure

C.check

D.B or C

答案及解析： D

题意：当你使用煤气时，要确保开着一扇窗子。make sure that..., check that...都可表示“确保……”。be sure that“确信……”，不符合题意。