the box and monitor vs. the box and the monitor


I have a question about ‘and’. Here are two sentences:

a. The box and computer monitor are red.
b. The box and the computer monitor are red.

Which is grammatically correct? I think ‘b’, but I don’t know how to explain why. Maybe… when we use ‘A and B’, the forms of A and B are same… if A is a noun phrase, B should be the same…? Please give me an answer.

Thank you in advance!

Interesting question! I feel the difference is: a) seeing ‘the box and computer monitor’ as a set of two things; b) two separate things. So, grammatically both are fine. aren’t they?