Câu 28: D
Ta có $\dfrac{1}{2}\ne \dfrac{2}{3}$
nên để song song: $k=\dfrac{-5}{7}$
Câu 29: C
$\dfrac{4}{7}\ne \dfrac{-1}{2}$
nên để song song: $\dfrac{2m+3}{5}=\dfrac{5m+2}{3}$
$\Leftrightarrow 5(5m+2)=3(2m+3)$
$\Leftrightarrow m=\dfrac{-1}{19}$
Câu 30: B
Để cắt nhau: $2m+1\ne 5m-3$
$\Leftrightarrow m\ne \dfrac{4}{3}$