In the following process, what is the throughput time $($also known as flow time$)?$

Step $1$

Step $2$

Step $3$

Step $4$

Resource $1$

Resource $2$

Resource $3$

Resource $4$

Activity

$12mi\frac{n}{?}$ unit

$10mi\frac{n}{?}$ unit

$15mi\frac{n}{?}$ unit

$10mi\frac{n}{?}$ unit

a$.15$ minutes

b$.10$ minutes

c$.47$ minutes

d$.12$ minutes