Which of the following sentences includes signal words that suggest a cause and-effect relationship between two events?

A. Darwin proposed that the species we know today are actually
descendants of ancient species that are now extinct.

B. For example, Darwin saw orchids that seemed to have a special
shape that would attract only very specific kinds of insects to
pollinate the orchids.

C. Because of natural selection, only the animals with the best
adaptations in any population survive and reproduce.

D. In addition to studying the adaptations of live animals on his
travels, Darwin also studied the fossils that he came across.