OG8 阅读 Q30

Choice C is the best answer. The lower graph, which shows the "tip moving away from salt surface," indicates that when the microscope tip was 15 nanometers from the surface, the force on the tip was approximately 0.75 micronewtons.
Choices A, B, and D are incorrect because the graph shows that when the microscope tip was 15 nanometers from the salt surface, the force on the tip was approximately 0.75 micronewtons, not 0 micronewtons (choice A), 0.25 micronewtons (choice B), or 1.25 micronewtons (choice D).