**Question:** S.I Unit of radioactivity is

A. Becquerel (Bq)
B. Curie (Ci)
C. Sievert (Sv)
D. Gray (Gy)

**Correct Answer:** A. Becquerel (Bq)

**Core Concept:** The S.I (International System of Units) units used for measuring radioactivity are based on the properties of radioactive substances.

**Why the Correct Answer is Right:** The Becquerel (Bq) is the S.I unit of radioactivity, named after Henri Becquerel, a French physicist who discovered radioactivity. One Becquerel is defined as the number of radioactive decays per second in one kilogram of pure uranium-235. This unit helps in standardizing measurements of radioactivity across different substances and experiments.

**Why Each Wrong Option is Incorrect:**

**Option B (Curie):** A Curie (Ci) is a traditional unit of radioactivity named after Marie Curie, a pioneering woman in the field of radioactivity. However, it is no longer considered an S.I unit and has been replaced by Becquerel (Bq).

**Option C (Sievert):** A Sievert (Sv) is a unit of absorbed dose of ionizing radiation. Although related to radioactivity, Sievert is not the correct answer because it measures the biological effect of ionizing radiation, not radioactivity itself.

**Option D (Gray):** A Gray (Gy) is a unit of absorbed dose of ionizing radiation, similar to Sievert. However, it is not the correct unit for measuring radioactivity.

**Clinical Pearl:** Understanding radioactivity and its measurement is crucial for medical students and professionals to understand the dosage of ionizing radiation administered to patients during medical procedures, ensuring optimal treatment and minimizing risks.