Order the statements to describe how clouds that collapse to form solar systems are formed. (place the first step at the top.)
a) Molecular clouds become gravitationally unstable due to various triggers such as supernova shockwaves or galactic collisions.
b) Gravitational collapse begins, causing the cloud to fragment into smaller clumps.
c) Protostars form within these clumps as they accumulate more mass from the surrounding material.
d) As the protostars continue to accrete mass, they heat up and eventually ignite nuclear fusion, becoming fully-fledged stars.