"嗯"is the correct one, while sometimes we type "恩" incorrectly on the Internet. And if someone call me on the street, I may say"什麼事嗎?"(什麼means "what"; 是means "thing"; 嗎 is a question word)

"期"is the second tone, so it should be pronounced as ding4 qi2.

有……用途、有……作用 Although sometimes they are subtitute to each other, there still exist subtle differences of these two. "有……用途" is usually filled in "practical " objects, like medical, while "有……作用" can be used for expression of "abstract" matters and concrete ones as well. For example: 1. 這種草藥有治療疾病的用途--This kind of herb can be used to cure sickness. 2. 大聲ˇ的噪音有嚇阻敵人的作用--Loud noise has the effect of scaring and hampering rivals.