## Core Concept
The guideline values for radioactivity in drinking water are crucial for ensuring safe consumption and minimizing health risks associated with radiation exposure. These guidelines are typically set by international and national health organizations. The core concept here involves understanding the acceptable limits of radioactivity, specifically gross alpha and gross beta activity, in drinking water.

## Why the Correct Answer is Right
The World Health Organization (WHO) and other health agencies provide guidelines for drinking water quality, including limits for radioactivity. For gross alpha activity, the guideline value is often set around 0.5 Bq/L, and for gross beta activity, it is about 1 Bq/L. These values are aimed at keeping the annual effective dose from drinking water below 0.1 mSv (or 100 μSv), a level considered safe for lifetime consumption. The correct answer reflects these guideline values.

## Why Each Wrong Option is Incorrect
- **Option A:** 1 Bq/L for gross alpha and 2 Bq/L for gross beta activity might seem reasonable but does not align with the commonly recommended guideline values.
- **Option B:** Similarly, 2 Bq/L for gross alpha and 4 Bq/L for gross beta activity are higher than the typically recommended limits.
- **Option D:** 10 Bq/L for both gross alpha and gross beta activity are significantly higher than the guideline values and could pose health risks.

## Clinical Pearl / High-Yield Fact
A key point to remember is that exposure to radioactivity, even at low levels, can accumulate over time and potentially increase the risk of cancer. Therefore, adhering to guideline values for radioactivity in drinking water is crucial for public health.

## Correct Answer Line
**Correct Answer: C. 0.5 Bq/L for gross alpha and 1 Bq/L for gross beta.**