The moisture holding capacity of the soil in a 100-hectare farm is 20 cm/m. The root zone depth of the crop is 1.0 m and peak rate to moisture use is 5 mm/day. The field is to be irrigated when 40% of the available moisture in the root zone is depleted. Calculate the frequency of irrigation.

• A. 16 days
• B. 18 days
• C. 20 days
• D. 25 days