Final answer:
The standing wave that is two wavelengths long has five nodes, including the endpoints; two end nodes and three interior nodes for each wavelength.
Step-by-step explanation:
The number of nodes, including the endpoints, in a standing wave that is two wavelengths long is five. This is because each wavelength in a standing wave pattern has two nodes at its ends and one node in the middle, making three nodes per wavelength. Since the wave is two wavelengths long, you have two end nodes and three nodes from each wavelength, resulting in a total of 5 nodes (2 end nodes + 2 middle nodes for each wavelength).