Slavery was () in Canada in 1833, and Canadian authorities encouraged the slaves, who escaped from America, to settle on its vast virgin land.

单项选择题The police are trying to () what really happened.

A．ascertain
B．assert
C．avert
D．ascribe

单项选择题(), you need to give all you have and try your best.

A．Being a winner
B．To be a winner
C．Be a winner
D．Having been a winner

单项选择题Gun control is a subject () Americans have argued for a long time.

A．of which
B．with which
C．about which
D．into which

单项选择题So sudden () that the enemy had no time to escape.

A．did the attack
B．the attack did
C．was the attack
D．the attack was

单项选择题Critics believe that the control of television by mass advertising has () the quality of the programs.

A．affected
B．effected
C．lessened
D．declined