That the repeated thermonuclear outbursts of U Scorpii, a recurrent nova system consisting of a white dwarf star and a companion star, have occurred on timescales as brief as eight years—shorter intervals between outbursts than have been observed in some other novae of its class—_____ to the white dwarf’s extremely large mass and rapid accretion of hydrogen from its companion star.
Which choice completes the text so that it conforms to the conventions of Standard English?
Explanation
Context: The true subject is "That the repeated thermonuclear outbursts...have occurred on timescales as brief as eight years" — a noun clause, treated as singular. The long em dash phrase is a distractor.
Answer: A) is due
Why? A noun clause ("That the outbursts have occurred...") acting as subject → singular verb "is." "Is due to" is the correct idiom.
Traps:
- B) due → not a finite verb; creates a fragment
- C) being due → non-finite; creates a fragment
- D) having been due → non-finite; creates a fragment