What are some factors involved with the strength of a drug?

Answer: B — Duration of Use, Purity and Quantity

The factors that affect the strength of a drug include not only the purity and quantity, but also the duration of use. A drug with high purity and high quantity does not necessarily result in greater strength if it is only used for a short period of time. Similarly, a drug with low purity and low quantity can still have significant strength if used over a longer period of time. Only considering either purity or quantity as the sole factors for strength would be incorrect, making options A, C, and D incorrect choices. Only option B takes into account all three factors that contribute to the overall strength of a drug.