The book is the most difficult ______ the three. [ ]

This machine is very easy ______. Anyone can learn to use it in a few minutes.

When did it happen? —At 5:55 a.m. ______ the morning of October 15.[ ]

“______ do you like? The yellow one ______ the white one?” “The yellow one.”

It is said that potato chips ____________ by mistake about a hundred years ago.