dinosaurs _________ millions of years ago.

Cụm từ "cause harm to" mang nghĩa là gây hại cho ai/cái gì. Trong câu này, "smoking" gây hại cho "your health". Do đó, đáp án đúng là "to".

This question tests knowledge of using articles. In this case, we need to choose the appropriate article before the ordinal number "third".

- "No article": Not using an article. This case is incorrect because "third" is an ordinal number and usually comes with an article.

- "An": "An" is used before a word starting with a vowel (a, e, i, o, u). "Third" starts with a consonant, so it's not suitable.

- "The": "The" is used to refer to a specific or defined object. In this case, "third person" is not specific, but only an order.

- "A": "A" is used before a word starting with a consonant. "Third" starts with a consonant and it is an ordinal number, so "a" is the correct choice. It means "you are the third person to ask me that".