1. Suppose one twin takes a ride in a spacecraft traveling at a very high speed toward a distant star and then returns, while the other twin remains on Earth. Compared to the twin who remained on Earth, the astronaut twin is:

a. Younger than Earth's twin.

b. the same age as Earth's twin.

c. Older than Earth's twin.

d. Ages cannot be determined from the information provided.

2. A proton hits an oxygen-18 nucleus, producing fluorine-18 and another X particle.

a. Neutron

b. Positron

c. Alpha particle

d. Electron

e. Gamma rays

3. How often are 20mm microwaves? a. 100 MHz b. 400 MHz c. 15 GHz d. 73 GHz

4.