In a single$-$core CPU that has no pipeline, it takes $1$ ns $($nanosecond$)$ to fetch the instruction, $1$ ns to decode the instruction, $1$ ns to fetch the operands from the registers, $2$ ns to execute the instruction, and $1$ ns to write the result back to the register. What is the minimum clock cycle length of this CPU, if we want to complete one instruction in every clock cycle?

Question $10$ options:

$1$ ns

$1+1+1+2+1=6$ ns

$2$ ns