Both C and E describe contrasts in behaviour; however, E requires us to make assumptions which cannot be strongly asserted based on the information provided.
E is incorrect because we cannot be sure whether the chimpanzees understand tool use. While chimpanzees and orangutans exhibit different behaviours when presented with a tool, we cannot then extrapolate from these behaviours to determine whether the primates understand tool use or not.
C is more strongly supported by the naturalist's statements as the orangutans pretend to ignore the tool when the zookeeper is around and then use it when the zookeeper leaves. This act of pretending to ignore the tool means that at least some nonhuman primates are capable of deception.
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