A binary sequence is an array of $0's $ and $1's$. The number of n-digit binary sequences which contain even number of $0's$ is:

Question

\[\begin {array} {1 1} (1)\;2^{n-1} & \quad (2)\;2^n-1 \\ (3)\;2^{n-1}-1 & \quad (4)\;2^n \end {array}\]