**Core Concept**
Moderately hard water is defined as water with a specific level of dissolved minerals, particularly calcium and magnesium ions. The hardness of water is typically measured in terms of the concentration of these ions, usually expressed as parts per million (ppm) or milligrams per liter (mg/L).

**Why the Correct Answer is Right**
The correct answer is determined by the specific concentration of calcium and magnesium ions in moderately hard water. Moderately hard water usually contains between 60 and 120 mg/L of calcium carbonate (CaCO3), which is equivalent to 30-60 mg/L of calcium (Ca2+) and 30-60 mg/L of magnesium (Mg2+). This range is considered moderately hard, as it is not as hard as very hard water (which contains higher concentrations of these ions) but still harder than soft water.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the specified concentration is too low for moderately hard water. Moderately hard water should have a higher concentration of calcium and magnesium ions.
**Option B:** This option is incorrect because the specified concentration is too high for moderately hard water. Very hard water would have a higher concentration of calcium and magnesium ions than moderately hard water.
**Option C:** This option is incorrect because it does not provide a specific concentration range for moderately hard water. Moderately hard water should be defined by a specific range of calcium and magnesium ion concentrations.

**Clinical Pearl / High-Yield Fact**
Remember that the hardness of water can affect the scaling of pipes and equipment in water treatment plants and households. Moderately hard water may require additional treatment to prevent scaling and maintain water quality.

**Correct Answer: C. 120 mg/L**