I （would must）think so?

題目：

I （would must）think so?
Will the new iphone cost a lot?--I ___think so.Apple's products are usually expensive
A.would B.must
選什麼?理由

解答：

在情態動詞表推測這個作用下,must是如果選擇了i must think so,那麼說明說話人是百分之九十地肯定新的iphone