Why are there eighteen elements?

Next Abhidharma-Samuccaya addresses why there are exactly eighteen elements.

ཅིའི་ཕྱིར། ci'i phyir is why. ཁམས། khams is element, རྣམས། rnams is plural ending, so we are dealing with elements.

བཅོ་བརྒྱད། bco brgyad is eighteen. ཁོ་ན། kho na is only, ཞེ་ན། zhe na is what does it mean.

Puzzling all this together and refining: Why are there only eighteen elements?

Next, this why is answered.