Final answer:
Active transport is the process that allows calcium to enter root cells.
Step-by-step explanation:
The process that would allow calcium to continue to enter the root cells is active transport. Active transport is the movement of ions or molecules across a cell membrane against their concentration gradient, requiring the use of energy in the form of ATP. In this case, plant root cells actively transport calcium ions from the low concentration in the soil to the higher concentration within the cell.
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