Final answer:
In a standing wave that is two wavelengths long, there are 3 nodes, which are located at the beginning, middle, and end of the wave.
Step-by-step explanation:
When analyzing a standing wave that is two wavelengths long, it is important to understand how nodes are distributed along the wave. A node is a point where the wave disturbance is zero, meaning there's no movement at that point.
Since standing waves on a string have nodes at each fixed end and at points that correspond to an integer number of half-wavelengths, a wave that is two wavelengths long will have nodes at both ends and at the one-wavelength mark. Therefore, there will be nodes at three positions: the beginning, the middle, and the end of the two-wavelength segment. This accounts for 3 nodes in total.
With this understanding, the correct answer to how many nodes, including the endpoints, are present in a standing wave that is two wavelengths long, is 3.