Problem $1$ Please choose the correct answer and also write down your solution.

$(1)$ What is the compatibility equation for the shown structure?

a$.{}_B={}_C$

e$.{}_B=\frac{9}{4}{}_C$

b$.{}_B=2{}_C$

f$.{}_B=\frac{4}{9}{}_C$

c$.{}_B=\frac{4}{5}{}_C$

g$.{}_B=\frac{9}{5}{}_C$

d$.{}_B=\frac{5}{4}{}_C$

h$.{}_B=\frac{5}{9}{}_C$