Which of the following is not true?
(a) Number of edge-disjoint Hamiltonian cycles in $K_7$ is $3$
(b) If $G$ is a simple graph with $6$ vertices and the degree of each vertex is at least $3$, then the Hamiltonian cycle exists in $G$
(c) Number of Hamiltonian cycles in $K_{4,4}$ = 72
(d) If $G$ is a simple graph with $5$ vertices and $7$ edges, then the Hamiltonian cycle exists in $G$
Please help me understand all the options.