Упражнение 5. Используйте наиболее подходящую форму глагола в скобках; если возможны оба варианта, объясните разницу в их употреблении

Пример: Before I960, astronomers (have not spotted, had not spotted) had not spotted stars that send out both light and sound waves.

In 1960, two astronomers at Mount Palomar Observatory (had discovered, discovered) a starlike object at the edge of the universe. The astronomers were astounded to discover that this object (emits, was emitting) as much light as a thousand galaxies. Today, we (use, are using) the name quasar, which (stands, stood) for quasi-stellar. Quasi-stellar (meant, means) “starlike object.” So far, more than three hundred quasars (had been found, have been found) in space. Although astronomers today generally (had agreed, agree) that quasars arе powerful, fast-moving energy sources, they (will contradict, contradict) each other’s theories about the origin of quasars. One group currently (has been believing, believes) that quasars (consisted, consist) of giant, swirling masses of gas. These astronomers (say, said) that the gas (collapsed, is collapsing) on itself all the time, which is why quasars (had generated, are generating) so much energy. Other scientists (were asserting, assert) that quasars (develop, have been developing) from explosions of matter and antimatter, just as science fiction writers (predict, have been predicting) for many years. When scientists (had conducted, conduct) experiments that (brought, bring) matter and antimatter together, an explosion (will occur, occurs). Until we (understand, understood) more about the birth and death of galaxies, we probably (will not discover, have not discovered) the real story behind quasars.