In a thick cylinder, subjected to internal and external pressures, let \[{{r}_{1}}\] and \[{{r}_{2}}\] be the internal and external radii respectively. Let u be the radial displacement of a material element at radius \[r,{{r}_{2}}\ge {{r}_{1}}.\] Identifying the cylinder axis as z axis, the radial strain component \[{{\varepsilon }_{r}}\] is:

• A. \[u/r\]
• B. \[u/\theta \]
• C. \[du/dr\]
• D. \[du/d\theta \]