The correct answer is that their cells are increasing in number and their cells are growing in size. Both processes contribute to the growth of an organism. As cells divide and reproduce, they increase in number, while individual cells also grow in size through the intake of nutrients and accumulation of cellular materials. Additionally, cells can differentiate into specialized cell types, further contributing to the growth and development of the organism. The statement "their cells are getting older" is not directly related to growth but rather refers to the natural aging process that all cells go through over time.
