The following sentences form a paragraph. The first and the last sentences of the paragraph are given. The rest of the sentences are numbered as P, Q, R and S. These four parts are not given in their proper order. Read the sentences and choose the alternative that arranges them in correct order.

1. Different types of terrorism have been defined by lawmakers.

P. Researchers began to distinguish different types of terrorism in the 1970s

Q. but by that point, modern groups had began to use more modern techniques.

R. Types differ according to what kind of attack agents an attacker uses

S. or by what they are trying to defend.

6. So, it became part of the efforts to understand how to counter it.