为什么汽车发动机要安装排气消声器？
参考答案：
降低发动机的排气噪声，并使高温废气能安全有效地排出
— We need a room for tonight, and I’d like to take a chance to check in without reservation.
— Don’t you know about the jammed hotels in this season? ______.
A．Better play it safe
B．Do as you please
C．No problem
D．Think nothing of it
各种外加剂有相应的代号，下列正确的（）
A、缓凝性高性能减水剂：HPWR-R
B、早强型高性能减水剂：HPWR-A
C、早强型普通减水剂：WR-R
D、标准型普通减水剂：WR-S
