天然气在空气中可燃性限界范围很窄，因此，在燃烧过程中对缺氧很敏感，同时减少了回火的危险性。
参考答案：对
—He should have come to join in the thorough cleaning.
—He _____. I don’t think it was his turn.
A．needn’t have to
B．needn’t have
C．didn’t have to
D．doesn’t need to
简述根据我国的国情，设计的“三步走”的现代化总体战略？
