In which of the following cases it is possible to obtain different results for call-by-reference and call-by-name parameter passing?

An unrestricted use of the "goto" statement is harmful because of which of the following reason(s):

How many substrings (of all lengths inclusive ) can be formed from a character string of length $$n$$? Assume all characters to be distinct. Prove your answer.

Is the class of regular sets closed under infinite union? Explain.