$1.$ Suppose that h : $\{0,1\}$n$+$n $\{0,1\}$n is a compression function. We can define the hash function H : $\{0,1\}3$n $\{0,1\}$n such that H$($M$)=$ h $($ A $||$ h$($B$)),$ for any M in $\{0,1\}3$n$,$ where A are the first n bits of M and B are the last $2$n bits of M; thus M $=$ A $||$ B$.$ Prove that if H has a collision then also h has one $($thus$,$ if h is CR then also H must be CR$).$