Scenario Biomedical Scientists Have Recently Developed an Anticancer Drug Delivery System

Scenario
Biomedical scientists have recently developed an anticancer drug delivery system that sneaks the anticancer drug into cancer cells before releasing it. They were able to do this by using two sets of liposomes. An anticancer drug (Doxorubicin (Dox) ) bound to DNA molecules was added into one set of liposomes, and a high concentration of ATP molecules were added to the other set of liposomes. The drug is inactive when bound to DNA molecules. Researchers inject both types of liposomes into cancer cells, and upon doing so their contents are released. The ATP molecules from one set of liposomes bind to the DNA molecules from the other set of liposomes) and activate the drug, thus inducing cell death and decreasing tumor size.
Mice with breast cancer tumors were treated with this liposome system to test the effectiveness of the drug. The scientists performed three trials-one with both types of liposomes (Dox + ATP) , one with just the drug (Dox only) , and one with a salt solution (control) . Examine the figure below and answer the following questions.
-What is the best explanation for the intermediate tumor size that results from treatment with just the drug (Dox only) compared to the control and treatment with both liposomes (Dox + ATP) ?

A) Without ATP, Dox becomes an energy source for cancer cells.
B) Dox actually does not require ATP to be activated.
C) When ATP is lacking, not as much Dox can enter the cell via facilitated diffusion.
D) There is likely to be some ATP in the cancer cells normally, which acts to activate Dox.