Consider the following class diagram that models a registration system. 1 Express the following constraints in OCL. 1. Invariants (a) A student cannot register in a section of a course where he is not enrolled. (b) A student cannot enroll in a course unless he passed all the prerequisites of this course. (c) A student must have at least a 60 grade in order to pass a course (d) A student cannot enroll in a course he already passed. (e) A student can enroll only in offered courses (that is, in courses with open sections). 2. Pre- and post-conditions of the method Section.register