There are only three articles in the English language. One is definite, and two are indefinite. "The" is a definite article, while "a" and "an" are indefinite articles. To answer your question, you need to find the sentence that correctly uses "a" and "an". "A" is used if the word following it begins with a consonant, while "an" is used if the word following it begins with a vowel. Looking at the options provided in your particular problem, you can see that answer choice a is the only one that follows all of the above rules, making it the correct answer.
Sloths are xenarthrans – their closest relatives include anteaters and armadillos. And, among other things, large, curved claws and powerful forelimbs for digging are common xenarthran traits. It's likely that the last common ancestor of two- and three-toed sloths shared these features, too.
You’d have to write that based on personal experience
