unbounded preceding

Database

unbounded preceding

For example ,the question is as follows:
Within ordered analytic functions,what is signified by the syntax element UNBOUNDED PRECEDING?
A the one row prior to the current row
B the entire partition before the current row
C the number of rows before the current row
D the entire partition before and after the current row
please tell me which is true and why is true and the meaning of the UNBOUNDED PRECEDING and why use UNBOUNDED PRECEDING?