Because some teachers do put in their bests to teach the students but some students, no matter how much you teach them, they won't learn and that's not the teacher's fault. Though, the teachers should find a way to make the student learn, if the student still refuses to learn the teacher cannot be blamed.
C a bird, mainly because a wall, a perch, and light o not have wings. the comparison is made because they want the reader to understand how love can fly
