Read the Following Scenario to Answer the Following Question(s)

Read the following scenario to answer the following question(s) .
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 (type A) ,and a high concentration of ATP molecules were added to the other set of liposomes (type B) .The drug is inactive when bound to DNA molecules.Researchers inject both types of liposomes (A and B) into cancer cells,and upon doing so their contents are released.The ATP molecules (from liposome type A) bind to the DNA molecules (from liposome type B) 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.

-Why did tumor size decrease in the Dox only experiment to a lesser extent than the Dox + ATP experiment?

A) ATP is actually the drug that is killing the cancer cells,not Dox.
B) Dox is always active in cancer cells,so it should be able to kill some cells without ATP.
C) When ATP is lacking,not as much Dox can enter the cell via facilitated diffusion.
D) There is some ATP in the cancer cells normally,which acts to activate Dox.