Cloning plants is straightforward while cloning animals is _____________ more complicated.

At Sam’s birthday party he had many guests, who were ________friends of his.

—How______do you attend the evening classes?—______,three times per week.

All the books written by Dickens are available in our library. You may borrow ___ you love. 

History is the best teacher. It _______records the development path of each country and foretells the future to us.