The hoop stress at a point on the surface of  a thin cylindrical pressure vessel is computed to be $30.0\text{ MPa}$. The value of maximum shear stress at this point is

1. $7.5 \text{ MPa}$
2. $15.0 \text{ MPa}$
3. $30.0 \text{ MPa}$
4. $22.5\text{ MPa}$