Let $I=(S,V)$ be a MAC. If an attacker can find $m_0{m}_1$ s$.$t$.S(k,m_0)=S(k,m_1)$ when the key $k$ begins with bit $0$ and ends with bit $1,$ can this MAC be secure$?$

A$.$ Yes, an attacker cannot generate a valid tag for $m_0$ or $m_1$ since it does not know the key.

B$.$ Yes, since key $k$ is chosen uniformly at random. The attacker cannot tell if the key begins with bit $0$ and ends with bit $1.$

C$.$ It depends on the details of the MAC.

D$.$ No$,$ this MAC can be broken using a chosen message attack.