
Passage: The early universe contained only the lightest elements: hydrogen and helium. Heavier elements such as carbon can only be formed in the nuclear reaction of a star and spread when the star explodes. A recently discovered nebula contains carbon formed billions of years ago, when the universe was no more than 1.5 billion years old.
Question: If the above statement is true, which of the followings must be true?
A. A.The earliest stars contained only hydrogen
B. B.When the universe was less than 1.5 billion years old, some stars had already formed
C. C.This nebula also contains hydrogen and helium
D. D.The carbon in this nebula later formed part of some stars
Answer: