in the context of r-trees which of the following statements is true regarding MBR (minimum bounding rectangle)?

a) the MBR of a node completely covers the area occupied by its child nodes
b) the MBR of a node is always a square shape
c) the MBR of a node is always smaller in size than the MBR of its parent node
d) the MBR of a node can overlap with the MBRs of its sibling nodes