7. The ......... are doing the exercise right now.
A. student B. wifes C. mans D. students
8. Where did you put the ........... ?
A. knives B. knifes C. a knife D. two knife
9. ............. are expensive.
A. Pianoes B.Piano C. Pianos D. The piano

If a word ends with a consonant followed by the letter "o", its plural is formed by adding either "s" or "es" at the end of the word. Whether we have to add simply "s" or "es" can be known through practice. Hence, it is important to familiarise ourselves with such words and their plural forms. 
The word "Piano" ends with the consonant "n" followed by "o". Its plural is formed by adding "s" to the end of the word. 
Hence, the correct option is Option B i.e Pianos. The other options are incorrect as they do not follow the plural formation pattern explained above. 
10. Some .................. came to arrest him
A. policeman B. policesman C. policemen D. policemans
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