Let $y[n]$ denote the convolution of $h[n]$ and $g[n]$, where $h[n]=(1/2)^n u[n]$ and $g[n]$ is a casual sequence. If $y[0]=1$ and $y[1]=1/2$, then $g[1]$ equals
1. $0$
2. $1/2$
3. $1$
4. $3.2$