Variables {q and r} are live at the entry point of block 1. P is dead because p is overwritten at the first statement and the statement 2 of block 1 uses this new value.And from line 2 of block 1, P is never used. So, p is dead.
Similar is the case for variable S, which is overwritten at line 2 and read at line 3 in block 1. So, S is dead the entry point of Block1.
U is never read in block 1, so U is dead before Block 1.
Before Block 2, variables R and U are being read so they are live at the entry point as well in the assignment line of block 2.
Similarly, In block 3, variables S and U are live at the entry point of block 3.
Now, if we talk about block 4, at the entry point of block 4, variables V and R are live.
Talking about block 3, at the exit point variables V and R are live.
The rule of liveliness states that if a block B does not use a variable V which is live after it's exit point, then this variable V is live also before the entry point of Block B.
Reference: https://www.youtube.com/watch?v=mPNZAUa54hs&list=PLFB9EC7B8FE963EB8&index=81
Since Variables V and R are live after Block B3, so they must be live before block B3.(V is not shown in the figure).
So, at entry point of below Blocks
B2-R and U are live
B3 - R , S ,U and V are live.
which is common between B2 and B3- R and U. (Ans).