防水层的作用主要是防止雨雪水向屋面渗透。( )
参考答案：对
________ the difficult maths problem, I have consulted Professor Russell several times.
A．Working out
B．Worked out
C．To work out
D．Work out
苯塔回流比是如何定义？回流比大小对操作有何影响？
