Final answer:
The critical turning point for cancer occurs when the mass contains about 1 million cells and angiogenesis occurs, allowing the tumor to grow beyond passive nutrient diffusion.
Step-by-step explanation:
The critical turning point for cancer when the mass contains about 1 million cells and angiogenesis occurs is the acquisition of sustained angiogenesis.
Angiogenesis is the process of forming new blood vessels, and it allows the tumor to grow beyond passive nutrient diffusion. Once angiogenesis occurs, the tumor can obtain its own blood supply and receive the necessary nutrients and oxygen to continue growing.