As regards the first question, the correct answer is LINGUISTS STUDY LANGUAGE AS A SYSTEM.
For example, Saussure studies language as a system of signs, Chomsky studies language as a system of rules, and Halliday studies language as a system of choices. They all agree in that language is a system.
As regards the second question, the correct options are:
- They are completely arbitrary.
- They are used to communicate.
- They are uniform and conventional.