A local hospital is trying to analyze the efficiency of its emergency room by tracking the number of patient complications per total patients treated during various shifts, which may differ in length. What characteristic of the u Chart makes it a potential fit for this scenario?

A It charts the number of defective units when the subgroup size is constant.

B It monitors the fraction defective when the subgroup size is variable.

C It represents rare events and counts opportunities between them.

D It charts the number of defects per unit when the subgroup size is variable or constant.

E $0\mathrm{.}108$