In ${\mathrm{C}}_4$  plants 
a) the bundle sheath cells may form several layers around vascular bundles.
b) bundle sheath cells have many chloroplasts.
c) intercellular spaces are absent between bundle sheath cells.
d) bundle sheath cells have thin cell walls, and they are permeable to gases.
How many statements are correct?

The large cells around the vascular bundles of the C₄ pathway plants are called bundle sheath cells. The leaves which have such anatomy are said to have ‘Kranz’ anatomy. The bundle sheath cells may form several layers around the vascular bundles; they are characterized by having numerous chloroplasts, thick walls impervious to gaseous exchange, and no intercellular spaces.