防水层的铺设要按先（）后（），先（）后（）的顺序连续作业。
参考答案：低；高；远；近
油液中混入的空气泡愈多，则油液的体积压缩系数β愈（）。
Reportedly on April 15, 2013 three people including a Chinese girl were killed in the Boston Marathon attacks when two bombs were _______, which shocked the whole world.
A．set about
B．set off
C．set up
D．set out
