The permissible levels of E.coli (number/ml) in potable water is

**Core Concept**
The permissible levels of E.coli in potable water are regulated to ensure public health safety from waterborne diseases. E.coli is a gram-negative bacterium commonly found in the human gut, and its presence in drinking water indicates fecal contamination.

**Why the Correct Answer is Right**
The World Health Organization (WHO) and the Environmental Protection Agency (EPA) set the maximum permissible level of E.coli in drinking water at 0 colonies per 100 milliliters (mL). This is because even a single E.coli colony can indicate fecal contamination, which increases the risk of waterborne diseases such as cholera, dysentery, and typhoid fever. The presence of E.coli in drinking water is often used as an indicator of the presence of other pathogens.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not specified, but typically, the permissible levels of E.coli in potable water are not in the tens of thousands.
**Option B:** This option is not specified, but it's unlikely that the permissible levels of E.coli in potable water would be in the millions.
**Option C:** This option is not specified, but it's not a realistic or safe level of E.coli in drinking water.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the presence of E.coli in drinking water indicates fecal contamination, which increases the risk of waterborne diseases. Regular water testing and treatment are crucial to ensure public health safety.

**Correct Answer: B. 0 colonies/100 mL.**