<span>A gradual increase in atmospheric carbon dioxide would warm earths atmosphere becuse carbon dioxide is <em>a greenhouse gas</em></span>
Because you cannot test it or observe it.
A scientific theory are created through the scientific method, in the scientific method you need to be able observe it and experiment.
If the Na+ voltage-gated channels in a muscle cell open, the Na (sodium) ions rush inside the muscle cell and makes the cell more positive as Na ions carry positive charge on it. Due to the entry of positively charged ions inside the cell, the cell becomes less negative and more positive. This is called depolarization of cell.
1.The presence of valves 2.the milking action of skeletal muscles 3. the veins as the muscles contract
