1. The causes of gamma-ray bursts throughout the universe, and how the gamma rays are actually produced, ________ until recently remained a mystery.

A. have
B. have been
C. has been
D. has

2. That the legal drinking age ________ lowered is a hot topic for debate in many states.

A. should have
B. which should
C. should be
D. should

3. Discovery of these ancient anthropic markings ________ our understanding of how these early humans interacted with their environment.

A. has broadened
B. have broadened
C. broaden
D. will have broadened

4. The rivalry between the two communities stems from ancient times and openly __________ to this day.

A. persist
B. which persist
C. persists
D. which persists

5. Despite the simplicity of their construction, the ancient systems ________ exhibit very complicated behavior.

A. find to
B. found to
C. are found to
D. were found to