对反渗透膜的性能有何要求？
参考答案：
1对水的渗透性要大，脱盐率要高
2机械强度要好
3能适应压力、温度和水质等的变化
4使用寿命要长。
In those days, we had no phones, so we had to keep in touch ________ letters often.
A．with
B．of
C．on
D．through
改良种用的鹿豚、姬猪
