Final answer:
Australopithecus afarensis' teeth are intermediate between ape and human teeth due to the absence of a diastema and the presence of a bicuspid structure in the lower premolars with one cusp larger than the other. Hence, the correct answer is 'both A and C.'
Step-by-step explanation:
The teeth of Australopithecus afarensis are considered intermediate between ape and human teeth. This comparison can be studied by looking at the various characteristics, such as the presence of a diastema, the size and projection of canines, and the bicuspids in the lower premolars. While the canines of Australopithecus afarensis were smaller than those of apes, they were still larger than those of modern humans. The lower premolars are bicuspid, with one cusp larger than the other, a feature that is unique compared to the purely large projecting canines of apes or the more uniform teeth of humans.
Option C states that their lower premolars are bicuspid, with one cusp larger than the other, which is correct for Australopithecus afarensis. Option A, which states there is no diastema, is another feature that is intermediate between apes, who typically have a diastema, and modern humans, who usually do not. Therefore, the teeth of Australopithecus afarensis appear intermediate because of both A and C: there is no diastema, and their lower premolars are bicuspid, with one cusp larger than the other.