Exercício: EFOMM 2007 - Synonymous sentence

In 'We not only missed our plane, we had to spend a night at the airport as well.', The synonymous sentence to the following statement is:

a) Not only did we miss our plane, we had to spend a night at the airport too (the right answer)

b) We did not miss our plane only, but we also spended a night at the airport (my answer)

Hello everybody! I wish i knew what the difference between letter A and B,and what the meaning of "as well" at the end of the phrase (if i wrote this text wrong,correct me!!) Thanks!!