Question

"...a useful thing...": A (an) is the indefinite article. The Indefinite articles are used appropriately before a word or set of words in the following alternatives, EXCEPT:
a) A train, an open book, an honest man.
b) A yard, a honor, a house, an owl.
c) A university, a headache, a year, an eye.
d) An egg, a child, an hour, an uncertain quantity."...a useful thing...": A (an) is the indefinite article. The Indefinite articles are used appropriately before a word or set of words in the following alternatives, EXCEPT:
a) A train, an open book, an honest man.
b) A yard, a honor, a house, an owl.
c) A university, a headache, a year, an eye.
d) An egg, a child, an hour, an uncertain quantity.

Answer 1

Exceto na letra A pois o artigo indefinido a usado em “a headache” está especificando uma doença/ um mal estar