单选题 For any positive integer n, n＞1, the "length" of n is the number of positive primes (not necessarily distinct) whose product is n. For example, the length of 50 is 3 since 50=(2)(5)(5)
Which of the following integers has length 3?
(A) 3
(B) 15
(C) 60
(D) 64
(E) 105

【正确答案】 E

【答案解析】[解析] 根据题目中的定义可知：
(A)的length等于1；
(B)的length等于2，因为15=3×5；
(C)的length等于4，因为60=2×2×3×5；
(D)的length等于6，因为64=2×2×2×2×2×2；
(E)的length等于3，因为105=3×5×7。