A variable x is said to be live at a statement $${S_i}$$ in a programif the following three conditions hold simultaneously:

i. There exists a statement $${S_j}$$ that uses x
ii. There is a path from $${S_i}$$ to $${S_j}$$ in the flow graph corresponding to the program
iii. The path has no intervening assignment to x including at $${S_i}$$ and $${S_j}$$

The variables which are live both at the statement in basic block 2 and at the statement in basic block 3 of the above control flow graph are

Which one of the following is TRUE at any valid state in shift-reduce parsing?

Which one of the following fields of an IP header is NOT modified by a typical IP router?

Suppose that everyone in a group of N people wants to communicate secretly with the N - 1 others using symmetric key cryptographic system. The communication between any two persons should not be decodable by the others in the group. The number of keys required in the system as a whole to satisfy the confidentiality requirement is