If you live in earth's northern hemisphere, on june 20th or 21st you experience summer solstice, which represents the official start of summer. this is the longest day of the year. what causes this change in day length?
a) the apparent counterclockwise movement of the earth.
b) the earth is orbiting closest to the sun at this time.
c) earth's tilt toward the sun is maximized at this time in the northern hemisphere.
d) the northern hemisphere is tilted toward the sun while the earth is orbiting at its closest point to the sun.

Choose the incorrect statement (a) the motor end plate membrane is only capable of generating graded potentials, but it is contiguous with the skeletal muscle sarcolemma, which can generate action potentials. (b) propagation of an action potential through the t-tubules into the sarcoplasmic reticulum triggers the release of ca++ into the cytoplasm of a skeletal muscle fiber. (c) the generation of suprathreshold endplate potentials in multiple muscle fibers could be due to the increased frequency of action potential generation at the axon hillock of a single motor neuron. (d) a greater amount of ach would be found in the presynaptic vesicles of a motor unit that fired several action potentials after exposure to botulinum toxin, than in a motor unit that fired several action potentials after exposure to the venom of the krait snake (which contains α-bungarotoxin).