Hey. The first sound is the "lub." This is what you hear once the mitral and tricuspid valves of the heart close, which are the valves between the atria and the ventricles! Super cool. It is your third answer choice.
Not, Viruses are not considered "alive" because they lack many of the properties that scientists associate with living organisms. Primarily, they lack the ability to reproduce without the aid of a host cell, and don't use the typical cell- division approach to replication.
The answer you are looking for is “organ system”
Carbon is released back into the atmosphere when organisms die, volcanoes erupt, fires blaze, fossil fuels are burned, and through a variety of other mechanisms.
