求助几道数学题

yingshun0331

If n is the product of the integers from1 to 20 inclusive, what is the greatest integer k for which 2k is a factor of n?
不知道怎么算，求高人讲解。
A.    10
B.    12
C.    15
D.    18（答案）
E.    20
Q28:
For any positive integer n, the sum of the first n positive integers equals [n(n+1)]/2.  What is the sum of all the even integers between 99 and 301?
求高人讲解。
A.    10,100
B.    20,200（答案）
C.    22,650
D.    40,200
E.    45,150
Q10:
A set of numbers has the property that for any number t in the set, t + 2 is in the set.  If –1 is in the set, which of the following must also be in the set?

I.    -3
II.    1
III.    5


A.    I only
B.    II only
C.    I and II only
D.    II and III only
E.    I, II, and III
答案说是D，可本人觉得是C。求高人帮助，到底答案是什么？