Of course, the truth is not so simple. This century, the work of cognitive psychologists has illuminated the subtle forms of daily learning on which intellectual progress depends. Children were observed as they slowly grasped--or, as the case might be bumped into- concepts that adults take for granted, as they refused, for instance, to concede that quantity is unchanged as water pours from short stout glass into a tall thin one. Psychologists have since demonstrated that young children, asked to count the pencils in a pile, readily report the number of blue or red pencils, but must be coaxed into finding the total. Such studies have suggested that the rudiments of mathematics are mastered gradually, and with effort. They have also suggested that the very concept of abstract numbers--the idea of a oneness, a twoness, a threenes that applies to any class of objects and is a prerequisite for doing anything more mathematically demanding than setting a table--is itself far from innate.
What does the passage mainly discuss？

A. Trends in teaching mathematics to children.
B. The use of mathematics in child psychology.
C. The development of mathematical ability in children.
D. The fundamental concepts of mathematics that children must learn.