问题补充：
---do you send e-mails to your pen friend? ------Once a month.
A. How long
B. How far
C. How much
D. How often
答案：
A. How long
B. How far
C. How much
(答案→)D. How often
解析：句意：你多久给你的笔友发一封电子邮件？一个月一次。How long多长；How far多远；How much多少（后跟不可数名词）；How often 多久。故选D。
考点: 考查疑问词的用法。
