In each of the following sentences there is one... - WAEC English 2004 Question

In each of the following sentences, there is one word underlined and one gap. From the list of words lettered A - D, choose the word that is most nearly opposite in meaning to the underlined word and that will, at the same time, correctly fill the gap in the sentences.
A gully, which is a natural phenomenon should not be mistaken for a tunnel, which is..........