Final answer:
Emily claims the arsenic is for killing rats, a common use of arsenic in pesticides and insecticides, despite its toxicity and health risks.
Step-by-step explanation:
Emily claims that the arsenic she buys from the druggist will be used to kill rats. Arsenic has historically been recognized for its use in pesticides, herbicides, and insecticides, and its toxic properties make it effective for these applications. However, arsenic is also known for its harmful effects on human health, leading to its decreased use in such applications today.