Lead compound X is active at 5 mg per kg but toxic at 8 mg per kg. After structural modification the product X' is active at 5 mg per kg and toxic only above 80 mg per kg. Which statement is correct?

1. AX' is more potent, because its toxic dose is higher
2. BX' is safe at any dose, so overdose is impossible
3. CThe modification widened the gap between effective and toxic doses from a factor of 1.6 to 16, and X' above its toxic dose still acts as a poison
4. DThe change means X' can no longer be used in chemotherapy