Why does matter (or the mass that makes it up) never "go away?"

Which statement is best described as a atom

The equation picture below shows which type of nuclear reaction u 235 + n x e 134 + sr 100 + 2n

Afusion reaction takes place between carbon and another element. neutrons are released, and a different element is formed. the different element is a) lighter than helium.b)heavier than helium.c)the same weight as helium.d)dependent on the element that reacted with carbon.