Ecrire le plus simplement possible :
$$A=\frac{5}{7}-\frac{14}{25}\times\frac{15}{49}\kern1cm
B=\left(-2\right)^5-3^4\kern1cm
C=\frac{\dfrac{3}{4}+\dfrac{1}{\strut3}}{2-\dfrac{\strut7}{3}}$$