In the Research on Na⁺ Uptake by Clam Cells,the Researchers

In the research on Na⁺ uptake by clam cells,the researchers focused on Na⁺ transporters.Why can't Na⁺ ions cross a cell membrane without a transport protein?

A) There is not a Na⁺ gradient without a transport protein.
B) Na⁺ is hydrophobic.
C) Na⁺ uptake requires active transport.
D) Na⁺ can diffuse directly across a membrane.
E) Membranes are hydrophobic.