Assume the notation $A_3,B_1,B_2,B_3,C_1$ denotes the tons moved from Supplier $A$ to Plant $3,$ Plant $1,$ and Plant $2,$ respectively. The diagram shows the only possible paths between Suppliers A$,$ B$,$ C and Plants $1,2,$ and $3$ with the associated cost per ton. The available supply at the $3$ suppliers and requirements at the $3$ plants are also shown in tons. Define the following decision variables:

$A_3$ : amount of shipping from supplier $A$ to plant $3$

$B_1$ : amount of shipping from supplier $B$ to plant $1$

$B_2$ : amount of shipping from supplier $B$ to plant $2$

$B_3$ : amount of shipping from supplier $B$ to plant $3$

$C_1$ : amount of shipping from supplier $C$ to plant $1$

What is the destination constraint for Plant $3?$

$A3+B3=300$

$B1+C1=600$

$B1+B2+B3700$

$A3+B2+C1=1400$