What is the primary time of application for a pre-emergence herbicide?

Paragraph for question 21 to 24 Herbicides are chemical substances used to control or kill unwanted plants (weeds). They can be classified into selective herbicides, which target specific plant species without affecting others for example a and non-selective herbicides, which kill all vegetation they come into contact with. Herbicides can also be categorized by their mode of action, such as contact herbicides, which kill only the plant parts they touch, or systemic herbicides, which are absorbed and transported throughout the plant, killing it entirely.