Production and cost — Economics STD 12 Humanities & Commerce — Question

When the amount of only one input is increased, while other inputs are held constant, the total product (TP) in the beginning increases at an increasing rate. This is the first phase of increasing marginal returns to the variable input.
Beyond a point, with increase in the variable input, total product starts increasing at a diminishing rate. This is the second phase of diminishing marginal returns to the variable input.
Eventually, further increase in the variable input causes the total product to decline. This is the third phase of negative marginal returns to the variable input. The following schedule illustrates the point:

Units of Land (Fixed input)	Units of Labour (Variable input)	TP	MP*	Phases
1	1	3	3	(Phase I) TP rises an increasing
1	2	7	4
1	3	12	5
1	4	16	4	(Phase II) TP rises a diminshing rate
1	5	19	3
1	6	21	2
1	7	22	1
1	8	22	0
1	9	21	-1	(Phase III) TP falls
1	10	20	-1