【正确答案】A: The strength and density of polymers increases with increasing the crystallinity of polymers, but the toughness of polymers decreases with an increase in the crystallinity of polymers. The crystallinity of polymers can be improved using processing methods, such as extrusion. In such doing, the amorphous polymers are oriented in the direction of drawing when these molecules are passing though a die, thus the crystallinty of polymers and in turn the strength ofpolymers is enhanced.