How many radioactive atoms are present in a sample that has an activity of 0.5 mCi and a half-life of 10 years? (1 curie = 3.7 × 10^10 decays/s)
a. 9.5 × 10^12 atoms
b. 8.4 × 10^12 atoms
c. 7.3 × 10^12 atoms
d. 6.5 × 10^12 atoms
e. 2.7 × 10^5 atoms
B
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Why must radio telescopes have very large dishes?
A. A large mirror is necessary to resolve radio-emitting images. B. The large dish is to compensate for atmospheric absorption of radio waves. C. Large radio telescopes are easier to repair than small instruments. D. Radio-emitting objects are very far from Earth, and so are very dim. E. Size is immaterial to quality observations.
In Copernicus' model of the solar system, the planets orbited the ________ in ________ orbits.
A. Sun; circular B. Earth; circular C. Earth; elliptical D. Sun; elliptical
The pregalactic blobs had masses similar to
A) dwarf galaxies. B) the Sun. C) the Earth. D) the Milky Way. E) open clusters of stars.
A river's suspended load and bed load may accumulate at its mouth and form a
a. sea arch. b. guyot. c. delta. d. sea mount.