Which of the following sentences is grammatically incorrect?

Question

Quizplus · Accepted Answer

"I′ve heard this spring is going to be unusually warm, we should start planting the garden soon" is grammatically incorrect because it is a run-on sentence.
Run-on sentences are when two or more complete sentences are written as one sentence. Run-on sentences can be fixed by adding a dash (-), a semicolon (;), or a colon (:) between the two sentences. You could also add a coordinating conjunction with a comma (, but) or make them two separate sentences with a period (.).
Any of these would be considered correct:
I′ve heard this spring is going to be unusually warm - we should start planting the garden soon.
I′ve heard this spring is going to be unusually warm; we should start planting the garden soon.
I′ve heard this spring is going to be unusually warm, so we should start planting the garden soon.
I′ve heard this spring is going to be unusually warm. We should start planting the garden soon.

Which of the Following Sentences Is Grammatically Incorrect