Practitioners of shifting cultivation usually begin the process of clearing a new field when

a) they are certain the last frost is pastb) the farmers vote to do soc) the soil of the old field is depleted of nutrientsd) seasonal and annual rainfall patterns shifte) the government tells them to do so

c) the soil of the old field is depleted of nutrients

Explanation:

Shifting cultivation is a traditional system of farming which involves the cultivation of a plot of land for 2 to 3 years, and then moving to another new virgin land, when nutrients on the old site have been depleted. The abandoned land is left to lie fallow, while it recovers and regains its fertility naturally.

Practitioners of shifting cultivation would normally start clearing a new field as the fertility of the old field becomes depleted. They abandon the old land when nutrients become depleted, and move to the newly cleared plots.