It is not always easy for the public to see _______ use a new invention can be of to human life.
A．whose B．which C．that D．what
He dressed up and went to the party as if ________.
A．was invited B．had been invited C．to be invited D．invited
After completing and signing it, please return the form to us in the envelope _______.
A．provided B．providing C．having provided D．provide
The book was written in 1946, _______ the education system has witnessed great changes.
A．since then B．when C．since when D．during which
He ______ be sitting at the gate, waiting for his lost son.
A．used to B．was used to C．could D．was accustomed
It is often recommended that children _______ chances to get close to nature.
A．must be offered B．should offer C．be offered D．are offered
