Answer
a science experiment, the controlled or constant variable is a variable that does not change. For example, in an experiment to test the effect of different lights on plants, other factors that affect plant growth and health, such as soil quality and watering, would need to remain constant. a dependant variable(opposite)would be the properties of the plants themselves, which the scientist is observing in relation to the changing light. These properties might include the plants' color, height and general health of the plant
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In developed countries , amount of food produced being increased is a positive effect of agricultural changes in the past century.
Agricultural changes in developed countries were set up such as application of fertilizers etc so as to achieve a maximum yield as the increasing population all over the world means increase in food production to cater for individuals and to prevent starvation and death.
Agricultural changes were also done to ensure goods produced have a very high quality.
The remaining options such as Desertification, Clearing of rainforests and contamination can be referred to as part of the negative effect of agricultural changes in the past century.
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This would be called a Parasitic Relationship.
Yes, also 50% is half marks so either way it’ll still be a pass!!