He hurried to the booking office only _____ that all the tickets had been sold out.[ ]

Have you heard about that fire in the market?— Yes, fortunately no one _____

Not until the early years of the 19th century ________ what heat was.[ ]

We cannot figure out ______ quite a number of students were infected with the flu.

下面与f字母含有不同发音的是 – _______.