Geologists once thought that the molten rock known as lava was an underground remnant of Earth's earliest days, sporadically erupting through volcanoes, but they now know that it is continuously created by the heat of the radioactivity deep inside the planet.

A)was an underground remnant of Earth's earliest days, sporadically erupting

B)had been an underground remnant of Earth's earliest days and sporadically erupted

C)was an underground remnant of Earth's earliest days, which sporadically erupted

D)would be an underground remnant of Earth's earliest days that sporadically erupted

E)was an underground remnant of Earth's earliest days, having sporadically erupted

1) How do you decide between was and had been?

2) Explaination says -(B) This choice unnecessarily and incorrectly changes the simple past "was" to the past perfect "had been," which is used only when describing the earlier of two past actions. Moreover, the use of "and" here equates the geologists' false understanding of lava with the fact that it sometimes erupts through volcanoes.

I don't understand both the parts. the use of verb tense as well as the meaning. i thought the sentence was talking about the lava sporadically erupting.

3) isn't sporadically erupting modifying earth's earliest days, since its right after the comma? Or am I now getting the whole concept of modifiers wrong?

OA is A

Please help