Question

which sentence should be inserted between sentences 5 and 6?
a due to the hot, flexible mantle under earths crust, the tectonic plates are usually moving.
b although scientists once believed them to be fixed in place, tectonic plates are quite flexible.
c the rigid boundaries of the tectonic plates are what scientists study when predicting earthquakes.
d erosion caused by the intense heat has made the tectonic plates perfect for the formation of volcanoes.

Explanation:

To determine the best - fitting sentence between sentences 5 and 6, we need to consider the context of tectonic plates. Option A provides a scientific fact about the movement of tectonic plates due to the hot, flexible mantle under the Earth's crust, which is a logical connection in a text about tectonic plates. Option B has an incorrect description as tectonic plates are not "flexible" in the sense described. Option C focuses on earthquake prediction which may not be a direct connection without proper lead - in. Option D has an unclear and incorrect statement about erosion and the formation of volcanoes related to tectonic plates.

Answer:

A. Due to the hot, flexible mantle under Earth's crust, the tectonic plates are usually moving.