Match the following: Some K+ channels remain open,
and Na+ channels rest.
1. Threshold stimulus
Voltage-gated Na+ channels open.
2. Depolarization
3. Hyperpolarization
Na+ channels are inactivating, and
voltage-gated K+ channels open.
4. Repolarization
Any stimulus at or above this level
will result in action potential.

the answer is "a.) well–developed vascular system."

explanation:

igneous rocks which formed by the crystallization of magma at the surface of the earth called extrusive rocks. they are characterized by fine-grained textures because the rapid cooling at or near the surface did not provide enough time for large crystals to grow.

before, trains worked using the steam engine. thus, it uses heat so it is crucial to apply heat balance. the steam engine absorbs energy during the stage of boiling water. so, this is the endothermic reaction. when the steam is used to work the piston, it has to release this energy through running the train. the excess energy is released as an exhaust to the chimney. this is the exothermic process because it releases energy to the surroundings.