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.
"Hello Musa" is a ...........opening for a friendly letter, but it is inappropriate for a formal letter

A. casual
B. serious
C. happy
D. suitable  

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Correct Answer: D