What is an element of geometry which has no dimension ? (line segment/ point/line)

In the figure what is the name of pair of angles which are constructed equal to make $l\parallel m$.

Choose the correct name from: Alternate Interior Angles / Corresponding Angles / Vertically Opposite Angles.

If $m \parallel n$ and $\angle \mathrm{ABC}=60°$ what should be the measure of $\angle \mathrm{DCB}$?

If $n\perp m$, $l\perp \mathrm{n}$ and the angle between lines $l$ and $m$ is $k°$ then find $k$.

Observe the figure:

If $\mathrm{BD}=5 \mathrm{cm}$ then the measure of $\mathrm{CD}=k \mathrm{cm}$?