A.  $\frac{1}{2}$

B.  $\frac{A_{50}^{25}.{\left(A_3^1\right)}^{25}}{{\left(A_4^1\right)}^{50}}$

C.  $\frac{1}{16}$

D. $\frac{C_{50}^{25}.{\left(C_3^1\right)}^{25}}{{\left(C_4^1\right)}^{50}}$

A.  $\frac{1}{2}$

 B.  $\frac{A_{50}^{25}.{\left(A31\right)}^{25}}{{\left(A41\right)}^{50}}$

C.  $\frac{1}{16}$

D.  $\frac{C_{50}^{25}.{\left(C31\right)}^{25}}{{\left(C41\right)}^{50}}$