After the physician taps your knee with a small hammer, your lower leg “jumps” in a reflexive response. The muscles of your leg then send messages back to the spinal cord to let it know that the leg has been extended. This two-way form of communication is called a(n) .

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the rocks are classified by the geological process they are formed by. the three main groups of rocks are igneous, metamorphic, and sedimentary. minerals are not they are classified under the chemical classes they are in. these groups include silicates, sulfates and carbonates.