Which of these is an agent of abrasion?
a. rock pieces
b. acid rain
c. high pressures
d. plant roots

A. Rock pieces

Explanation:

Abrasion is the breaking down and wearing away of rock material by the mechanical action of other rocks. Three agents of physical weathering that can cause abrasion are moving water, wind and gravity. None of those are agents of abrasion. But Rock pieces are a result of abrasion.

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the principles and empirical processes of discovery and demonstration considered characteristic of or necessary for scientific investigation, generally involving the observation of phenomena, the formulation of a hypothesis concerning the phenomena, experimentation to test the hypothesis, and development of a conclusion that confirms, rejects, or modifies the hypothesis.