问题
单项选择题
Greg sells gaskets. On three sales, Greg has received commissions of $385, $70, and $190, and he has one additional sale pending. If Greg is to receive an average (arithmetic mean) commission of exactly $220 on the four sales, then the fourth commission must be:（）.
A. $135
B. $155
C. $220
D. $235
E. $645
答案
参考答案：D
解析：
For the average of four commissions to be $220, the total must be 220×4=880. 880 minus the sum of the three listed commissions (385+70+190=645) equals $235.