A block has velocity function $\vec{v}\left(t\right)$ that increases with time along the $+\text{x}$ direction. What must be true about the net force on the block?

The net force must alternate direction since velocity is positive.

The net force must be in the $+\text{x}$ direction because the acceleration is positive.

The net force must be upward because the speed is changing.

The net force must be zero because the object is already moving.