21. According to reading passage, Maya Fishbach says GW190412 is “an exceptional event” because __________.
(A) it’s the first collision between two black holes that detected in human’s history.
(B) more than one observatory detected the event and named collaboratively.
(C) it is a collision of black holes with a strange mass discrepancy.
(D) it enables astrophysicist to know more about the Universe’s heavy chemical elements.