Answer
a. companion cells
b. sieve elements
Work Step by Step
Both companion cells and sieve elements are part of the phloem, the tissue that transports sugars and other nutrients throughout the plant. These two cells are both alive when mature.
On the other hand, the two component cells that make up the xylem, vessel elements and tracheids, are dead when mature, but their cell walls remain in place. Water is transported through these dead cells; their cell walls also serve to support the plant.
