Which of the following subsets of $\displaystyle P_{2}$ are subspaces? The set of all polynomials of the form:

a. $\displaystyle a_{2}t^{2}+a_{1}t+a_{0}$, where $\displaystyle a_{1}=0$ and $\displaystyle a_{0}=0$

b. $\displaystyle a_{2}t^{2}+a_{1}t+a_{0}$, where $\displaystyle a_{1}=2a_{0}$

c. $\displaystyle a_{2}t^{2}+a_{1}t+a_{0}$, where $\displaystyle a_{2}+a_{1}+a_{0}=2$