The Aurora Borealis, or Northern Lights, occurs near Earth's magnetic poles ______ charged particles from the sun collide with atmospheric gases.
Which choice completes the text so that it conforms to the conventions of Standard English?
- A
(empty)
- Bcheck_circle
, when
- C
when
- D
,
Explanation
", when" introduces a subordinate clause explaining the timing/cause. ", " creates a comma splice; " when" without comma is acceptable for restrictive use but here a comma fits the descriptive sense; " " is a run-on.