详解
Choice A is the best answer. That Moore's group was surprised to observe salt stretching is most directly suggested by the last sentence of the third paragraph: "But scientists don't expect this superplasticity in a rigid, crystalline material like salt, Moore says."
Choices B, C, and D are incorrect because the cited lines don't support the idea that Moore's group was surprised to observe salt stretching. Instead, they explain how the group happened upon their observation (choice B), the measures the group took to investigate the stretching further (choice C), and how common salt is in nature (choice D).