Question

complete the text with the better conjunctive adverb. most people are aware of the eight basic blood types: a, ab, b, and o, each of which can be
egative\ or \positive.\ _______, each of these eight types can actually be classified into millions of varieties based on the antigens that coat red blood cells.
howevever
after all

Explanation:

"However" is used to introduce a contrast. The first - part states common knowledge about basic blood types, and the second part shows a more complex classification. "After all" is used to emphasize a reason or conclusion, which doesn't fit the context here.

Answer:

However