What is the constraint for node $2$ in the following maximal flow problem?

a$.$ X$12<=4$

b$.$ X$12+$ X$13-$ X$23=0$

$.$ X$12-$ X$23-$ X$24=0$

d$.$ X$12+$ X$23+$ X$24=0$

$+0$

$23)$ What is the objective function in the following maximal flow problem?

a$.$ MAX Xia

b$.$ MAX Xa

c$.$ MIN Xi

d$.$ MAX X$12+$ Xis