How to get the radius to get the volume of the figure? I have this statement:

In the tube of the attached figure, the segment AC is tangent at A to
  the circumference inside. The volume of the tube can be determined, if
  it is known that:
(1) The measurement of the AC segment is 5 cm. (2) The length of the
  tube is 80 cm.


With $2)$ I can't because i need to get the radius, 
With $1)$ Maybe there is a relationship to get the radio, but I would miss the length.
With $1), 2)$ together, I think that is possible, but my problem is that, I don't know how to find the radius given the tangent $AC = 5$
PD: The correct answer must be 1, 2 together
Any hint is appreciated.

The tough part is finding the area of the annulus.  Obviously, it is the area of the larger circle minus the area of the inner circle, which is $\pi(OC^2-OA^2)$.  But, by the Pythagorean Theorem, we know that $OC^2-OA^2=AC^2=25$.  Therefore, the area of the annulus is $25\pi$ cm and thus the volume of the pipe is $25\pi\cdot 80=2000\pi\mathrm{\ cm^3}$.
