Fill in the blanks with the most appropriate words.
A preposition is a word which comes before a noun or a pronoun and establishes a relationship between the elements of a clause or words. Option C)by-on is correct as by and on indicate usage and a medium to travel respectively. The other options are wrong as in indicate inside something. the second blank can not take by as it is used to show how something is done. These uses don't fit in the given question. The correct answer is C)by-on.