**Explanation:**

In civil engineering, the stiffness of a beam is an important factor to consider when designing structures. The stiffness of a beam determines its ability to resist deflection under load. A beam with higher stiffness will experience less deflection than a beam with lower stiffness.

The stiffness of a simply supported beam can be determined by its span-to-depth ratio. The span-to-depth ratio is the ratio of the length of the beam (span) to its overall depth. It is denoted by the symbol L/d.

The ACI 318 Building Code Requirements for Structural Concrete provides guidelines for the maximum span-to-depth ratio for simply supported beams. According to ACI 318, the maximum span-to-depth ratio for normal cases is given by the equation:

L/d ≤ 20

This means that the span of the beam should not exceed 20 times its overall depth in order to satisfy the stiffness requirements for normal cases.

Let's take an example to understand this concept. Suppose we have a simply supported beam with a span of 10 meters and an overall depth of 0.5 meters. The span-to-depth ratio for this beam would be:

L/d = 10/0.5 = 20

Since the span-to-depth ratio is equal to the maximum allowed value of 20, this beam would satisfy the stiffness requirements for normal cases.

However, if we increase the span of the beam to 12 meters, the span-to-depth ratio would be:

L/d = 12/0.5 = 24

Since the span-to-depth ratio exceeds the maximum allowed value of 20, this beam would not satisfy the stiffness requirements for normal cases.

Therefore, the correct answer is option C (20). The span-to-depth ratio should not exceed 20 for normal cases in order to satisfy the stiffness requirements of a simply supported beam.