In an experiment to determine the percentage of humus content in a soil, the following results were obtained:
mass of dish = 10 gm
mass of dish and oven-dry soil = 50 gm
mass of dish and soil after heating = 40 gm
Calculate the amount of humus in the soil sample.

• A.10 gm
• B.40 gm
• C.50 gm
• D.60 gm
• E.90 gm