A firm which bottles shampoo s

A firm which bottles shampoo selects some filled bottles for examination.The procedure used is to select a random starting point x(xth bottle filled) and every bottle at an interval of y is then chosen for examination.What is this type of sampling known as?
A、Multi-stage
B、Random
C、Systematic
D、Stratified

【参考答案及解析】
This is the definition of systematic sampling.