Which of the following is a reason why sex workers are at higher risk of contracting HIV/AIDS and other sexually transmitted infections?

A) Many of them are injection drug users.
B) Most of them do not have orgasms when offering sexual services to their clients.
C) Many of them require regular blood transfusions.
D) Most of them indulge in anal sex rather than oral sex.

Injection drug use is a significant risk factor for HIV/AIDS and many other sexually transmitted infections. Sex workers who also inject drugs are at an increased risk due to the sharing of needles and other drug use equipment. Option B is irrelevant to the risk factors for contracting HIV/AIDS and other sexually transmitted infections. Option C is incorrect because blood transfusions, when done properly, do not increase the risk of contracting HIV/AIDS or other sexually transmitted infections. Option D is a generalization and not necessarily true for all sex workers, and either anal or oral sex can increase the risk of contracting HIV/AIDS and other sexually transmitted infections.