A muscle cramp is a sudden, painful contraction of a muscle, during which movement is difficult. What is happening at the cellular level during a muscle cramp? A.
A lack of calcium causes the sarcomeres to lock together, preventing relaxation.
B.
The proteins in the myofibrils become denatured, preventing relaxation.
C.
Myosin and actin do not fully detach, so the muscle cannot relax.
D.
The motor neuron does not send a signal, so the muscle cannot relax.

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many phylogenetic trees have a single lineage at the base representing a common ancestor. a lineage that evolved early from the root and remains unbranched is called basal taxon. when two lineages stem from the same branch point, they are called sister taxa.

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explanation:

the correct answer, according to e2020, is the second choice there is an occluded front in region a and a cold front in region b.