冬季启动推土机应该注意什么？
参考答案：先低速运转并预热发动机及部位，待水温升至50℃，油温升至40℃时方可缓慢加大油门。
No matter how much time I spend planning a class, little things still _____ go wrong. So I had no perfect class and I reflected all the time.
A．deliberately
B．obviously
C．originally
D．occasionally
图中的等腰直角三角形表示三棱镜．光线垂直于一个面入射，在底面上发生全反射，由此看出棱镜的折射率不可能是( )．
A．1.7
B．1.8
C．1.5
D．1.36
