问题如下：
There are two companies in an economy, Company A and Company B. The default rate for Company A is 10%, and the default rate for Company B is 20%. Assume defaults for the two companies occur independently.
What is the probability that either Company A or Company B defaults?
选项：
A.2%
B.28%
C.8%
D.30%
解释：
Pr(A∪B) = Pr(A) + Pr(B) - Pr(A∩B) = 10% + 20% - 2% = 28%.
P(AB)=10%*20%=2%，这个2%为什么这么做呢？
