The lengths of a large stock of titanium rods follow a normal distribution with a mean $(\mu)$ of $440$ mm and a standard deviation $(\sigma)$ of $1$ mm.What is the percentage of rods whose lengths lie between $438$ mm and $441$ mm?
1. $81.85 \%$
2. $68.4 \%$
3. $99.75 \%$
4. $86.64 \%$