Robert is writing an instruction manual for a line of power tools. Though the manual is in English, ...

Robert is writing an instruction manual for a line of power tools. Though the manual is in English, it will be used by people from many different countries. What should Robert do to improve the readability of the document?
A.    He should use mostly medium-length sentences to show relationships among ideas.
B.    He should use only short sentences in order to explain the concepts clearly.
C.    He should use only long or medium-length sentences as the processes are complex.
D.    He should primarily use short sentences, but vary sentence length as required.