Which word best describes the hot, weak, inner part of the upper mantle, which can flow?

a.
asthenosphere

b.
lithosphere

c.
crust

d.
core

Asthenosphere is the answer of this question. It best describes the all above mention properties. 

Asthenosphere is the the upper layer of the earth's mantle, below the lithosphere, in which there is relatively low resistance to plastic flow and convection is thought to occur.

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