show interest 还是interests?为什么?

1.show interest in = 对某事/人感兴趣;是固定词组 :
She showed interest in ballet as a child and her parents sent her to ballet school at age five.
The boy showed no interest in the food which was laid before him, but jumped off the table and went out to play.
Their company showed much interest in our product.
和下面句子比起：
It is not in the interest of their company to do business under such disadvantageous conditions.
就是2.in someone's interest/interests 对 X 没有什么利益 = to someone's advantage 的意思了
为了谋人的利益:把某人的利益放在心上 have someone's interests at heart
还有：3.interest cost = 利息费用

show interest.

are interested in 感兴趣=show interest in
不加s