When you are using gas, ________ that one of the windows is open.
A.be sureB.make sureC.checkD.B or C在线课程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。