What is the primary difference between benign and malignant tumors?

• A. Malignant tumors are always larger than benign tumors.
• B. Benign tumors are cancerous, while malignant tumors are not.
• C. Benign tumors invade surrounding tissues, while malignant tumors do not.
• D. Malignant tumors can metastasize, while benign tumors remain localized.

Answer: (D)