TOEFL Listening: TPO-TOEFL阅读TPO - N3479O534584BC7PN$

According to paragraph 7, Peter Sheldon's studies demonstrated which of the following about trilobites? A. They underwent gradual change over a long time period B. They experienced a number of discontinuous transitions during their history C. They remained unchanged during a long period of environmental stability D. They evolved in ways that cannot be counted for by either of the two competing theories.