The answer is A as it is stating an opposite claim.
A restrictive adjective clause is not set off with commas. The correct answer would be the first option. Restrictive clause is a clause that cannot be omitted in the sentence for it will change the idea of the whole sentence. It is the opposite of non-restrictive clause--a clause that serves as an additional information that is set off with commas.
<span>Convenience sampling can produce biased research results. TRUE.</span>
