If n is the product of the integers from 1 to 20 inclusive, what is the greatest integer k for which 2^k is a factor of n?
A. 10
B. 12
C. 15
D. 18
E. 20
麻烦老师给我辨析一下图1和图2,关于图1那题当时选错之后机构老师给我解答的是40 to 44 million years 是一个时间区间,不能用estimate at 因为 at 有精确到某个点的意味。那么图2的正确答案是不是因为3.47 billion years 是一个精确的年份所以要用 at 。。