Determine whether or not the following claims are true for all regular expressions $r_1$ and $r_2$. The symbol $≡$ stands for equivalence of regular expressions in the sense that both expressions denote the same language.
(a) $(r_1^*)^*≡r_1^*$.
(b) $r_1^*(r_1+r_2)^*≡(r_1+r_2)^*$.
(c)$(r_1+r_2)^*≡(r_1^*r_2^*)^*$.
(d)$(r_1r_2)^*≡r_1^*r_2^*$.
