The National Day of the United States falls on ______.

A．The American senators are elected in rotation for six years.
B．The American president is elected in November each leap year.
C．An American vice president does not function unless through the chance of the president's death.
D．The American president's veto can't be overridden by the Congress.

A．transformational generative grammar
B．Bloomfieldian linguistics
C．systemic-functional grammar
D．the Prague School