OG8 阅读 Q26

Choice D is the best answer. The first sentence of the sixth paragraph identifies "electrostatic forces, perhaps good old van der Waals interactions" as the potential cause of the initial attraction between the microscope tip and the salt.
Choices A, B, and C are incorrect because the first sentence of the sixth paragraph clearly identifies the potential cause of the initial attraction between the microscope tip and the salt as van der Waals interactions, not as gravity (choice A), nano-imprinting (choice B), or surface tension (choice C).