Susan: Many rare plants have provided cures for various diseases. Since many plants have been lost to extinction, it is likely that there could have been cures for many more diseases. That is why it is important for us to manage our resources responsibly and prevent plant extinction. Robert: But plants are not our resources, they are part of the same environment we inhabit, and we are not superior to them. We must treat plants as the co-equal inhabitants of this planet that they are. Which one of the following is an issue about which Susan and Robert disagree? ['whether humans should prevent plant extinction', 'whether animals should be prevented from extinction', 'whether plants should be treated humanely', 'whether humans should exploit plants for human benefit']