______forms a natural boundary between Mexico and the United States.A．The Rio Grande River

The phrase "kicked around" （paragraph 4) could be best replaced by ______A．inventedB．form

This river forms a natural _______ between China and KoreaA.boundaryB.stringC.spotD.zone

The Nature paper suggests______may be an influential factor affecting malaria.A．warmer cli

The collapse of manners in Britain can be ascribed to______.A．natural evolution of human b

Fevers cause ______.A．prostaglandins to be formed

A new form. of energy must be found _____ oil supplies will soon be used up.