The production manager is developing a linear program to determine a minimum cost $6-$months production plan which takes into account production and inventory costs. The manager defines the following decision variables:

Ri $=$ the number of units produced in month i during Regular Time $($i$=1,2,....,6)$

Vi$=$ the number of units produced in month i during Over Time $($i$=1,2,....,6)$

Si$=$ the number of units held in inventory at the end of month i $($i$=1,2,....,6)$

Which of the following equations represents the $$balance constraint$$ for month $4($i$.$e$.$ establishing the 'flow'$/$relationship between production, demand, and inventory levels during month $4)$

Group of answer choices

R$4+$ V$4+$S$4=1900$

S$3+$ R$4+$ V$4$ S$4=1900$

S$4+$ R$4+$ V$4$ S$5=1900$

$50$ R$4+75$ V$4+8$ S$4=1900$

None of the above