A．Because the professor may need them from time to time.
B．Because they are very precious and valuable.
C．Because the professor hopes they are available to all the students.
D．Because they are already reserved by some students.
A．Humans' social structure is the most complicated.
B．Humans are able to cooperate with each other.
C．Prediction is the key to humans' survival.
D．Humans are sensitive and motivated.
A．Boiling hot water.
B．Slightly hot water.
C．Room temperature water.
D．Icy cold water.