Which of the following statements about airfoils and wings is/are true?

To evaluate each of the statements regarding airfoils and wings:

1. **An airfoil only feels two-dimensional flow, while a wing feels three-dimensional flow**: This statement is generally true. An airfoil can be considered to be experiencing two-dimensional flow as it is analyzed under the assumption that it extends infinitely in the spanwise direction. In contrast, wings, which are finite and have a span, experience three-dimensional flow effects including tip vortices and variations along the span of the wing.

2. **An airfoil is the cross-section of an infinitely long wing**: This statement is also true. An airfoil represents the shape of a wing in a two-dimensional sense, essentially being the profile of the wing at any given point along its length, assuming the wing extends infinitely.

3. **We use lowercase letters in the coefficients to refer to airfoils**: This statement is not necessarily true. In aerodynamics, uppercase letters (e.g., CL for lift coefficient, CD for drag coefficient) are typically used in the context of airfoils and wings. Lowercase letters may have other applications but are not standard for referring to airfoil coefficients.

Considering the assessment of the statements, the only statements that are true are the first two. Therefore, the final conclusion is:

**Answer: None of the above.**