16) He works as ..... electrician in a power plant.
Explanation: Indefinite articles 'a' or 'an' are used with the names of profession. The noun 'electrician' begins with a vowel 'e' so use 'an' in this sentence.
17) Rex has ..... terrible toothache.
Explanation: Indefinite article 'a' will be used with noun complement terrible.
18) After this exam you have .... whole afternoon free to explore the city.
Explanation: We are talking about a particular afternoon, so use the definite article 'the'.
19) The potatoes are 25 rupees .... kilo.
Explanation: Indefinite article 'a' is used with numerical expression.
20) ..... Smiths live in New York.
Explanation: Definite article 'the' is used with the family names.