The distance around the circle is called a semicircle.

Which of the following pairs of lines can be parallel?

$1.$ Two tangents to a circle                                            $2.$ Two diameters of a circle

$3.$ A chord of a circle and a tangent to a circle.             $4.$ Two chords of a circle 

A _____ is a line in the plane of a circle that intersects the circle at two points. 

Coplanar circles that have the same centre are called _____.

The diameter of a circle is of length $6 \mathrm{cm}$. Its radius is equal to _____ $\mathrm{cm}$.

PQ is tangent to the circle with centre O at point A on the circle, then $\angle OAQ=$_____$°$.

A part of the circumference of a circle is called an _____.

Name the line segment $\overline{PQ}$ in the figure given below.

In the figure, $OB$ is tangent to circle with centre $A$. Find $\angle \mathrm{AOB}$