Which of these fits gap 20?

Directions: Complete this passage by selecting the most suitable option for the corresponding gap.
All rocks ______ (17) _______ formed in the same way.
One kind of rock, called “sedimentary rock,” is formed by sediment. Sandstones and lime–stones are _____ (18) _______ sedimentary rocks.
Since they are ______ (19) _______ in layers which are called “strata,” these rocks are called “stratified” rocks.
The tiny particles of which such rocks are made are usually rounded, since the sharp edges and corners ______ (20) _____ worn off while the particles were being rolled along stream beds, washed by the waves, or blown by winds.