It’s definitely not B or C, because MacReady is proven human in the blood test and we’re with him the rest of the movie, which means he couldn’t have been infected or assimilated at any point.
40 YEARS LATER, AND FANS OF THIS TERRIFYING, CLASSIC SCI-FI HORROR MOVIE, ARE STILL DYING TO KNOW, WHICH ONE WAS IT? B.) MacReady; A.) Childs; C.) Both; D.) Neither