For any arbitrary motion in space, which of the following relations are true

$a) {\overrightarrow{v}}_{\text{average}}=\frac{1}{2}[\overrightarrow{v}\left(t_1\right)+\overrightarrow{v}\left(t_2\right)]$

$b) {\overrightarrow{v}}_{\text{average}}=\frac{[\overrightarrow{r}\left(t_2\right)-\overrightarrow{r}\left(t_1\right)]}{(t_2-t_1)}$

$c) \overrightarrow{v}(t)=\overrightarrow{v}(0)+\overrightarrow{a}t$

$d) {\overrightarrow{a}}_{\text{average}}=\frac{[\overrightarrow{v}\left(t_2\right)-\overrightarrow{v}\left(t_1\right)]}{(t_2-t_1)}$

(The ‘average’ stands for average of the quantity over the time interval $t_1$ to $t_2$  )