How do you ensure that a third-degree Bezier curve is a straight line segment?

Question

Suppose that we have a Bezier curve of degree $3$. How can I ensure that the curve will be a straight line?

So as an example take this figure

I think that we can guarantee that the red line is a straight line if the weights of $b_1$ and $b_2$ are always zero with given parameter $t \in \space [0,1]$.

Answer 1

The cubic Bezier curve will be a straight line when all the 4 control points are collinear.