答案：B

解析：

Choice B is the best answer. Cruden presents the finding that "magma actually has quite a low viscosity and is relatively runny" as the information that changed scientists' understanding of how long it took magma to form the continental crust. The discussion of magma's viscosity suggests that scientists originally believed the continental crust took millions of years to form because they thought magma was thicker than it is and could not have flowed through the cracks in the lower crust, and that it instead solidified quickly and then moved very slowly through the mantle and lower crust as a solid.