The role of the president has evolved over time becausea. the needs of the nation have changed. b. the role of the executive branch has changed. c. the role of congress has changed. d. the needs of the chief of staff have changed.

A, the needs of the nation have changed.

Explanation:

By the way, the answers to the whole Unit Test Review are here:

1: A.

2: D.

3: B.

4: A.

5: A.

6: B.

7: C.

8: B.

9: A.

10: A.

11: C.

12: A.

13: D.

14: B.

15: C.

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two jet streams that directly affect the formation of weather in the continental part are a polar jet and a subtropical jet. they are responsible for the transportation of weather systems, which directly affect people. the polar front is the boundary between cold air from the north pole and warmer equatorial air. both the polar jet and the subtropical jet blow from the west to the east, and follow the boundary between cold and warm air.

the strongest jet streams are polar jets, while subtropical jets are somewhat weaker.

the right answer is c. the polar jet stream is generaly stronger than the subtropical jet stream.