The picture shows a group of muscle cells in the heart. all of these muscle cells beat in unison to push blood in timing with the rhythm of the heart, at the direction of nearby nerve cells.

these muscle cells could best be called a
a) organ.
b) tissue.
c) macromolecule.
d) organ system.

i think that d. wind is the answer.   i missed the question when i answered a. currents.

answer: ice crystals in air

the water from the water body or another source of water gets evaporated as water vapor as an effect of heat on water. these water vapor rises in the upper layers of the earth atmosphere under high atmospheric pressure. the atmospheric pressure of these water molecules starts to decrease due to the effect of cold air. the invisible water vapor condenses into visible water droplets and ice crystals.these ice crystals and water droplets being less denser than the surrounding air floats on air as clouds.