Express each of these sets using a regular expression.
a) The set of strings of one or more 0s followed by a 1
b) The set of strings of two or more symbols followed by three or more 0s
c) The set of strings with either no 1 preceding a 0 or no 0 preceding a 1
d) The set of strings containing a string of 1s such that the number of 1s equals 2 modulo 3, followed by an even number of 0s