Which of the following is NOT a limiting factor for the number of possible compounds? all elements have a different atomic number
many elements are unable to react with other elements
some elements are less reactive than others

Sorry i think the answer is b

Ch4 and ph3 are both covalent since they are composed both of non-metals.    ch4 is non- polar because of  london forces and has a geometry of tetrahedral  ph3 , on the other hand, is umbrella shaped polar and has dipole-dipole intermolecular force (imf) . since ph3 is polar, it has a stronger imf