filling forward conditional result

I have a DataFrame with columns A and B. Now I want to produce column C like this:

       A   B   C
index  
 1     0   50   NaN
 2     1   60   60
 3     0   40   60
 4     0   30   60 
 5     1   40   40

C gets the value of B if A==1 in this row. Then this value is kept in coming rows until next time A==1.How can I do this in vectorized fashion?