D - an increase in tree cover in the tundra.
Since the other answer got deleted, i'll just answer it for you.
a sinoatrial node is the main pacemaker that begins each heartbeat located in the junctions of superior vena cava and the right atrium. It signals from the node that spread through the atria and determines the heart rates.
Missense mutations.
Hope this helps!
Thin, flexible barrier around a cell; regulates what enters and leaves the cell
- contains the cytoplasm (all interior cell organelles and the cytosol) - allowing the chemical reactions in the cell to occur
<span>- semipermeable (or selectively permeable) - allows certain substances in, keeps others out </span>
<span>- offers limited protection</span>
