Based on Monster group and GPT:

First, let's look at what each option means:

- A. PI * r * r * h * 1/3: This formula is the correct formula for the volume of the cone, but the multiplication operator has A lower priority than division, so 1/3 is computed first and the result is 0, resulting in a calculation error.

- B. 1/3 * PI * r * r * h: This formula is the correct formula for the volume of a cone, and the priority of the operator is correct, so the volume of a cone can be calculated.

- C. PI * r * r * h / 3: This formula is the correct formula for the volume of the cone.

- D. PI * r * r * h * 1.f / 3: This formula is the correct conical volume formula, but to avoid integer division, 1.f is used to convert 1 to a floating-point number, guaranteeing the result to be a floating-point number.

- Therefore, choice A is an expression that cannot calculate the volume of a cone, and the other three choices are OK.

On another point, the formula used when calculating the volume of a cone should be 1/3 * PI * r * r * h, not 4/3 * PI * r * r. Precision problems caused by integer division can be avoided if 4/3 is assigned directly to floating-point variables, such as:

```
float const PI = 3.1415926f;
float const R = 10.0f;
float const H = 20.0f;
float const k = 4.0f / 3.0f;
float const volume = k * PI * R * R * R;
cout << "圆锥体积：" << volume << endl;
```

assigns 4/3 directly to a floating point variable k, so that the calculated volume of the cone is accurate.