Habits, whether good or bad, are gradually formed. When a person does a certain thing again, he is impelled(,)  by some unseen force to do the same thing repeatedly; thus a habit is formed. Once a habit is formed, it is difficult, and sometimes impossible, to get rid of. It is therefore very important that we should pay great attention to the formation   of habits. Children often form bad habits, some of which remain with them as long as they live. Older persons also form bad habits as long as they live, and sometimes become ruined by them.. There are other habits which, when formed in early life, are of great help. Many successful men say that much of their success has something to do with certain habits in early life, such as early rising, honesty and thoroughness . Among the habits which children should not form are laziness, lying, stealing and so on. These are all easily formed habits. Unfortunately older persons form habits which ought to have been avoided. We ought to keep from all these old habits, and try to form such habits as will prove good for ourselves and others. Habits, whether good or bad, are formed  _  ,   _   everyone knows.
day by day; and gradually; as gradually; that century after century; as
Question: Habits, whether good or bad, are gradually formed. When a person does a certain thing again, he is impelled(,)  by some unseen force to do the same thing repeatedly; thus a habit is formed. Once a habit is formed, it is difficult, and sometimes impossible, to get rid of. It is therefore very important that we should pay great attention to the formation   of habits. Children often form bad habits, some of which remain with them as long as they live. Older persons also form bad habits as long as they live, and sometimes become ruined by them.. There are other habits which, when formed in early life, are of great help. Many successful men say that much of their success has something to do with certain habits in early life, such as early rising, honesty and thoroughness . Among the habits which children should not form are laziness, lying, stealing and so on. These are all easily formed habits. Unfortunately older persons form habits which ought to have been avoided. We ought to keep from all these old habits, and try to form such habits as will prove good for ourselves and others. Habits, whether good or bad, are formed  _  ,   _   everyone knows.
A. day by day; and
B. gradually; as
C. gradually; that
D. century after century; as
Answer: