"The study is comparing the safety and efficacy of a new MDR-TB drug, delamanid, with the decades-old TB drug isoniazid for preventing active MDR-TB disease in children, adolescents and adults at high risk who are exposed to adult household members with MDR-TB." What does the word efficacy mean in the context of this sentence?
A)failing to accomplish any desired results
B)an estimate of the cost of a rare commodity
C)located in a far away from a useful location
D)the ability to produce a desired result or effect