What are the 3 main classes of elements?

What does the process of natural selection involve

Consider the nuclear equation below. 239/94 pu—-> x+ 4/2 he. what is x?

The twister and runaway train are two coasters at the same amusement park. both coasters start at the same height. the coaster for the twister is twice the mass of the coaster for the runaway train. which roller coaster has greater gravitational potential energy at the start of the ride?