Transportation Network

$15$A$.)$ In the network representation above the arrows between the plants and cities represent the possible flows of electricity from plants to cities. If $(12)$ represents the FLOW from Node $1$ to Node $2,(14)$ represents the flow from Node $1$ to Node $4,$ and so forth, which of the following represents the total INFLOW to Node $3?$

A$.)(13)+(23)+(32)+(31)$

B$.)(13)+(23)$

$15$B$.)$ The given capacity for the two warehouses is $0.$ How should this be interpreted?

A$.)$ The warehouses are only used to transfer inventory from plants to customers.

B$.)$ The warehouses should not be used.