In each of the following sentences there is one... - WAEC English 2009 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.
This is quite surprising, you cannot differentiate between the authentic and the.............receipt