Which of the following could replace 'tremble' as used in the passage?

Directions: Read the following passage carefully and select the most suitable option according to the corresponding number in the passage.
A star is a heavenly (1) body which shines by its own light. Planets, as you know, shine only by light reflected (2) from the sun. The planets shine with a steady (3) light and the stars twinkle. This twinkling is caused by desturbances (4) in the air between the star and the earth. The unsteady air bends the light from the star, which then appears to tremble (5). Stars differ greatly (6) in size, density and temperature. At one end (7), there are stars called “red super-giants,” which are many times (8) larger than our sun (which is also a star). At the other end, there are stars called “white dwarfs,” some of which are about the size of our earth.