Give a regular expression for the following languages. (a) A = {w: w has two O's } where = {0,1} (b) B = {w: w ends in "txt" } where = {a,b,...,z} (c) C = {w: w : w starts with "aa" and ends with any number of "z"s } where = {a,b,...,Z} (d) D = {w: w is a string representing an even binary number} where = {0,1}