Which statement is true?
A: A system of equations consists of two or more linear equations that must be true at the same time.
B: A system of equations consists of exactly two equations that must be true at the same time.
C: A system of equations consists of two or more equations that must be true at the same time.
D: A system of equations consists of two or more equations with at least one linear equation such that all equations must be true at the same time.