Antidepressants work by stopping the breakdown of norepinephrine, dopamine, and serotonin. They're more difficult for people to take than most other antidepressants because they interact with prescription drugs, nonprescription drugs, and some foods.
In the following question, out of the four alternatives, select the alternative which will improve the bracketed part of the sentence. In case no improvement is needed, select "no improvement".
The shoes were very expensive; I couldn't afford (to buying) them.