What do the xiongnu the uighurs the khitan and the jurchen have in common?

The xiongnu, the uighurs, the khitan, and the jurchen were all able to gain large amounts of valuable goods from China, often as disguised "protection money" that kept them from attacking China, and they all established nomadic empires to the north of China.

Neurotransmitters are messengers responsible for communication between neurons. they are released from synaptic vesicles and are received by receptors.  the communication is referred to as neurotransmission. the following statement is true about neurotransmitters: a. they're stored in synaptic vesicles in the axon.

It depends on the surface of the levels. the rougher the surface, the greater is the frictional force. for rougher surfaces, you would need to exert greater force to move the object. the concept is that, you must overcome the frictional force in order to bring the object into motion. this is the reason why we add lubricants to the joints so they could easily move. this answer was on