Bad Qubits — Learn Quantum by Doing

Multiple choice

In Shor's period-finding subroutine, after applying the modular-exponentiation oracle U_f to get sum_x |x>|a^x mod N>, measuring the OUTPUT register collapses the input register to which kind of state?

A uniform superposition over all 2^n inputs, unchanged by the measurement
A uniform superposition of equally-spaced integers x0, x0+r, x0+2r, ... (an arithmetic progression with spacing r)
An equal superposition of just |0> and |r>
A single computational basis state |x0> with no superposition remaining