A Planaria worm is cut horizontally in he middle into two halves P and Q such that the part P contains the whole head of the worm. Another Planria worm is cut vertically into two halves R and S in such a way that both th cut pieces R and S contain half hed each. Which of the cut pieces of the two Planaria worms could regenerate to from the complete respective worms ?
A. only
B. only R and S
C. P,R and S
D. P,Q,R and S
