Irene Pepperberg worked with an African gray parrot named Alex. Her research revealed that _________________.
a) Alex was able to make requests and to answer simple questions about objects
b) Alex showed no evidence of cognitive abilities, which isn't surprising given his brain was the size of a walnut
c) Alex's linguistic abilities were limited to the "parroting" of phrases that he heard Irene speak
d) Alex was capable of addition, subtraction, multiplication, and division problems, showing an innate understanding of numbers