单项选择题

High-grade written paper is frequently obtained from cotton rags.

A．High-grade
B．written
C．frequently
D．obtained
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单项选择题
When the concentration of calcium in the blood is too low, the parathyroid glands began to secrete the hormone parathormone.
A．concentration
B．too
C．began
D．secrete
单项选择题
Fossils of plant that have been extinct for fifty million years have been found inlarge deposits of amber near the Baltic Se
A．A. plantB. forC.fifty million yearsD.near
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