The first time that astronomers observed both gravitational waves and electro-magnetic waves from the same event, what they were observing was:
a. the merger of two black holes with masses dozens of times the mass of our Sun
b. a supernova explosion in one of the closest galaxies to us
c. the collapse of an entire cluster of stars into one big black hole
d. the spiraling toward each other of two neutron stars
e. a binary star system where a giant star is overloading a white dwarf with too much material