The number of defects in the first five cars to come through a new production line are 9, 7, 10, 4, and 6, respectively. If the sixth car through the production line has either 3, 7, or 12 defects, for which of theses values does the mean number of defects per car for the first six cars equal the median?

I. 3

II. 7

III. 12

A. I only

B. II only

C. III only

D. I and III only

E. I, II, and III

先把9，7，10，4，6相加得36

if 3,mean=6.5, median=6.5

if7, mean>7,median=7

if12, mean=8,median=8

so, I and III only.