Consider the following sets. $A=\{([xyz])in{R}^3$:$5x-6y-4z=0\}B=\{([xyz])in{R}^3$:$([xyz])=([30-7])+([0-7-5])$ and $-6x-9y-4z=0\}C=\{pin{P}_3$:$p(x)=x^3+a{x}^2+bx+c$ where $a,b,c$ are real numbers $\}$ Select all of the following which are true $A$ is a subspace of $R^3.B$ is a subspace of $R^3.C$ is a subspace of $P_3.$