Why are spectral lines useful in classifying stars?

What is the border between layers a through f and layer g called?

How have citizens in aralsk adapted to the changes brought about by the shrinking of the sea?

What's a recharge area? a. the part of an aquifer where groundwater meets a lake or stream b. the part of an aquifer where surface water reaches the water table c. the part of an aquifer that's located between two aquicludes d. the part of an aquifer that's located at a lower elevation