TOEFL Reading: ETS-TOEFL阅读机经 - UT12B9JSZD38GSP5L$

In paragraph 2, why does the author mention the information about calcium ions A. To explain Koherig's theory that amphibian eggs developed at least two protective outer membranes B. To explain that the calcium in a hard shell is not poisonous when the egg is laid on land C. To explain why, according to Kohring, a hard shell evolved D. To explain why only the outer membrane stored calcium