Ans: B
$\begin{array}{rcl}
\dfrac{\text{the total cost of the mixture}}{\text{the total weight of the mixture}} & = & \text{the cost of the mixture per kg} \\
\dfrac{140x + 315y}{x + y} & = & 210 \\
140x + 315y & = & 210x + 210y \\
105y & = & 70x \\
\dfrac{x}{y} & = & \dfrac{2}{3} \\
x : y & = & 2 : 3
\end{array}$
$\begin{array}{rcl}
\dfrac{\text{the total cost of the mixture}}{\text{the total weight of the mixture}} & = & \text{the cost of the mixture per kg} \\
\dfrac{140x + 315y}{x + y} & = & 210 \\
140x + 315y & = & 210x + 210y \\
105y & = & 70x \\
\dfrac{x}{y} & = & \dfrac{2}{3} \\
x : y & = & 2 : 3
\end{array}$