我个人觉得那个袜子的combination不止45种而是55种 因为45种都是2双不同的袜子，如果是2双相同的袜子的这种情况没有包括在45种情况内。

A firm produces three types of shoes and ten types of socks. As a promotion, it plans to offer two pairs of socks with each pair of shoes. For each pair of shoes, the two pair of socks can be the same or different. How many ways can the three items be packaged?

A) 300.

B) 30.

C) 135.

D) 33.

Your answer: C was correct!

This is a case of the multiplication rule (shoes) combined with a combinination formula (socks). When considering the socks, order is not important. Therefore, we use the combination formula to arrive at (10!)/(8!(2!))= 45 ways to combine 10 types of socks two at a time. After realizing that two socks are combined with one of three different types of shoes, we simply use the multiplication rule. (45)(3)=135 combinations are possible