Single or Plural?

Which one is correct?

a) A line was drawn between A and B as well as between C and D

b) Lines were drawn between A and B as well as between C and D

In comparing a) and b), determining the correct one depends on what the writer's intent is. Both a) and b) are grammatical, but both also have different meanings.

a) A line was drawn between A and B as well as between C and D

In this example, there is one line drawn between point A and point B. There is also one line drawn between point C and point D. In total, there are two lines described. However, the two lines are between totally different points.

I also would recommend adding a comma, so that the sentence reads:

a) A line was drawn between A and B, as well as between C and D

We add a comma, because commas connect independent clauses with dependent clauses. "A line was drawn between A and B" is the independent clause. "as well as between C and D" is the dependent clause. It is also grammatical to add a comma at the same spot in example b).

As for what example b) means:

b) Lines were drawn between A and B as well as between C and D

In example b), there is an unspecified number of lines being drawn. There is an unspecified number of lines drawn between point A and B. There is also an unspecified number of lines drawn between point C and point D.

In conclusion, the writer must know how many lines are being used. If there are only two lines total, where one line is between A and B, and one line is between C and D, then example a) is "correct". If there are multiple lines in use, then we could select example b) as correct. Once the writer decides what they want to convey, they may pick a) or b) or write another as needed.