For the gear system, we have chosen such a representation that: the large gear rotates six times when the small one rotates fifteen times. How many times must the large gear rotate for the small one to rotate $100-\text{times}$?
(A) 30
(B) 35
(C) 40
(D) 45
(E) 50
A5 The rotational frequencies of the large and small wheels are in a direct proportion. The coefficient for the small wheel is $\frac{100}{15}=\frac{20}{3}$, so the rotational frequency of the large wheel is $6 \cdot \frac{20}{3}=40$.