Bending your head back until it hurts is an example of

the correct answer is - ability to find a mate

the correct answer is:

a population changes in response to a change in the environment.

igneous rocks are formed by solidification of cooled magma (molten rock), with or without crystallization, either below the surface as intrusive (plutonic) rocks or on the surface as extrusive (volcanic) rocks.  

the composition depends on the source magma. there are almost a thousand types of composition found till now.  

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the answer to the blank provided is “an autosomal gene that is required for the expression of genes on the x chromosome”.

testis-determining factor, abbreviated as td, which is more popularly known as sex-determining region y or sry protein, is a dna-binding protein or  gene-regulatory protein/transcription factor, encoded by the  sry gene. it is the gene responsible for the initiation of male gender determination in humans.