Pick the answer which fills the gap/s using the best and clearest English.'If you ... ... the only girl in the world,And I ... ... the only boy,Nothing else ... ... matter in the world today (...)'(Song lyrics by Clifford Grey, 1916)

A conditional sentence is based on the word 'if'. There are always two parts to a conditional sentence – one part beginning with 'if' to describe a possible situation, and the second part which describes the consequence. For example: If you work, I'll pay you.