Please explain why the answer to 5.2 is A, and why is B wrong.
I said B because "double-decker" is non-essential thus could be surrounded with commas. I also thought that if you have 2 adjectives "enormous" and "double-decker", that you would need a comma between them.
Reason being is because a comma is essentially a pause in a sentence. Sometimes that isn't the case. However, ninety percent of the time its a pause simply think about where a pause should be in any sentence.
