Directions: In each of the following sentences, some parts of the sentence have been jumbled up. You are required to rearrange these parts which are labeled as P, Q, R, S to produce the correct sentence. Choose the proper sequence from the alternatives provided. 

The green revolution, which is

P. undoubtedly increased the

Q. of a high-yielding variety of seeds and fertilizers

R. productivity of land considerably

S. often characterized by the introduction