Just a few years ago, biologist and medical personnel were surprised to learn that fetal cells can escape and enter the mother's blood. how can these cells be used in fetal testing?

links? do you have a link? i don't see anything?

the correct answer is option d.

an appositive is a noun, noun phrase or noun clause which follows another noun or noun phrase in order to describe it or name it another way. they are usually used between commas, brackets or dashes.

the example sentence with the correct use of commas is the following:

her uncle, the man over there in the red shirt, owns the hardware store in town.