If \([z]\) denotes the greatest integer less than or equal to \(z\) and \([z] = -1\) which of the following statements must be true?
A. \(z = -1\)
B. \(-2 \leq z < -1\)
C. \(-2 < z \leq -1\)
D. \(-1 \leq z < 0\)
E. \(-1 < z \leq 0\)
Answer: D
Source: GMAT Prep