The half-life of a radioactive isotope is 10,000 years. This isotope and its breakdown product do not normally occur together before molten rock cools and becomes solid. You analyze a volcanic rock and find 25 percent of the original amount of
radioactive isotope remains, and 75 percent has decayed to its daughter element. How long ago did the rock solidify?
A) 2,500 years ago B) 20,000 years ago C) 50,000 years ago D) 10,000 years ago
Answer: B
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A falling tide is called a(n):
a. flood tide. b. spring tide. c. neap tide. d. ebb tide. e. mixed tide.
Which of the following will NOT develop into a gamete?
a. spermatogonium b. polar bodies c. oocyte d. spermatid e. secondary spermatocyte
What happens when the resting potential reaches threshold voltage?
A) Voltage-gated sodium channels close. B) Voltage-gated sodium channels open. C) The sodium-potassium pump is turned on. D) Resting potassium channels close. E) Voltage-gated potassium channels open.
What determines the direction of the voltage change that occurs at the postsynaptic membrane?
A. The type of neurotransmitter B. The number of ion channels on the postsynaptic membrane C. The type of second messenger system used D. The type(s) of ions that travel through opened channels