已知函数f（x）=ln（e^x+1）+ax，（a＜0）

（Ⅰ）若函数y=f（x）的导函数是奇函数，求a的值；

（Ⅱ）求函数y=f（x）的单调区间。

Microwave communication uses high-frequency (71) waves that travel in straightlines throughthe air. Because the waves cannot (72) with the curvature of the earth, they can be (73) only over short distance. Thus, microwave is a good (74) for sendingdata between buildings ina city or on a large college campus. For longer distances, the wavesmust be relayed by means of"dishes" or (75) . These can be installed on towers, highbuildings, and mountain tops.

A．repeaters

B．radars

C．telephones

D．antennas