a) ask A to extend the deadline. Look for email template online. There are plenty
b) tell B that you have an offer in hand but you are very interested in B. They will ask you when you have to decide by, and usually they will accommodate.
這兩樣都是非常正常的,跟本不存在什麽不好意思。在網上查查這樣的email template 都很多。絕對不要say yes to A then back out. that is considered very unethical.
我剛剛經曆了同樣的情況。I told A to extend the deadline by one week, and when I went in for interview i told B I have an offer in hand, and asked them when is the latest they can decide. B is a company with 100k employees, but they still sped up the process. They usually do.