Now more and more people ________ sending e-mails ________ letters.

A．prefer to; to writing

B．prefer to; to write

C．prefer; to writing

D．prefer; to write

等精度观测了五边形内角各两个测回，一测回角中误差m_β=±40"，试求：
(1)五边形角度闭合差的中误差；
(2)欲使角度闭合差中误差不超过±50"，求观测角的测回数。