Job Offer...need help from 老土和各位大虾们！

好不容易拿了个大公司 A 的Offer, 很激动，马上签了字，will start next Monday.

可是前天就得到另一家大公司 B 的second interview,昨天recuiter 说，他们已决定要我，工资，benefits都比A 公司好很多，而且，我觉得B company 更Stable, 可是，要经过pre-employment physical test, and drug test.我还没有得到正式的Offer.

The controller just called me and gave me an offer on the phone, he said that he will ask HR to give me a formal written offer today, but today is Sat. it may not happen as he might not get the HR guy on the phone.

I decide that I will work for A company until I get the formal offer from B company and pass all the tests.

I am worried if I take time off to go to those tests, then A company will not be happy.

What is best solution for me right now?? thanks very much!!!