Heroin became the drug of choice for many opioid users

A. around 1900, because it could be injected.
B. during the 1920s, because it was easier to smuggle the small amounts needed for an effective dose.
C. during World War II, when supplies of raw opium were cut off.
D. in the 1960s, because users believed that it was more pure than other opioids.

B