急问几道GRE数学题!都是绿皮上的,本人数学不大好,185.20For which of the following pairs of integers is the least common multiple of the integers minus their greatest common divisor the greatest?　　A.3,12 B.5,6 C.10,20 D.11,12 E.15,30275.34If 1 is the greatest common divisor of thepositive integers a and b and if a/b-1=13/8.Then a+b=?A.11 B 13 C20 D21 E29158.30Which of the following cannot be expressedas the sum of either two or three consecutive positive integer?A.17 B18 C42

急问几道GRE数学题!
都是绿皮上的,本人数学不大好,
185.20
For which of the following pairs of integers is the least common multiple of the integers minus their greatest common divisor the greatest?
　　A.3,12 B.5,6 C.10,20 D.11,12 E.15,30
275.34
If 1 is the greatest common divisor of the
positive integers a and b and if a/b-1=13/8.Then a+b=?
A.11 B 13 C20 D21 E29
158.30
Which of the following cannot be expressed
as the sum of either two or three consecutive positive integer?
A.17 B18 C42 D56 E65
266.17
If n is an integer,then the units digit of
n^2 cannot be?
A0 B2 C4 D5 E6