Hệ phương trình nào sau đây vô nghiệm ?

$A.\left\{\begin{array}{l}x-2y=5\\ -\frac{1}{2}x+y=-\frac{5}{2}\end{array}\right.$

$B.\left\{\begin{array}{l}x-2y=5\\ -\frac{1}{2}x-y=3\end{array}\right.$

$C.\left\{\begin{array}{l}x-2y=5\\ \frac{1}{2}x+y=3\end{array}\right.$

$\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}D.\left\{\begin{array}{l}x-2y=5\\ \frac{1}{2}x+y=3\end{array}\right.$