Because if we're simultaneously moving/manipulating two different variables it can be very hard to determine why one variable is changing and why the other is changing. While this can be done with more complicated experimental designs, it usually is the case that scientists will only manipulate one variable and not two different ones.
B. Club moss
Explanation:
This is because club moss is an seedless evergreen plants that have scale-like leaves.
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Answer:
They are necessary for co-ordination.
Explanation:
The both systems brings about co-ordination, thought there are some basic differences between them. Many organs in an animals system have organs to carry out activities like locomotion, digestion, respiration and excretion. The activities of each organ system must be co-ordinated with those of other systems; and with conditions in the external and internal environments of the animal.
Unlike the endocrine system, the nervous system works rapidly. Messages are carried mainly as electric impulses along nerves triggering immediate response from target organs.
The hypothalamus is the important link between these two systems.
