The following six sentences come from a paragraph. The first and the last sentences are given. Choose the order in which the four sentences (PQRS) should appear to complete the paragraph:
S1: There are a number of bad habits which poor readers adopt.
S6: Young children and very poor readers often point with a finger at each word in turn.
P: Of course, there must be vigorous mental activity.
Q: But extra body movements, such as pointing with the fingers or moving the lips, do not help reading.
R: In efficient reading, the muscles of the eye should make the only external movement.
S: Most of these involve using extra body movement in the reading process.