A．To mix pulp with water.
B．To keep the shape and thickness of the paper.
C．To keep the paper strong enough.
D．To dry the paper quickly.
A．The cost of making paper.
B．Large machines and paper-making.
C．The development of paper-making.
D．How to make paper by hand today.
A．The continents and ocean floors move violently.
B．The earth is as permanent as it was.
C．The surface of our planet is always in motion.
D．The crust of the earth moves frequently.