Bad Qubits — Learn Quantum by Doing

Multiple choice

The lesson establishes the proven containment chain P subset of BPP subset of BQP subset of PSPACE subset of EXP. The genuinely nontrivial upper bound is BQP subset of PSPACE. What is the key idea behind that proof?

BQP problems are undecidable, so they vacuously lie in PSPACE
Each output amplitude is a sum over exponentially many computational paths that can be enumerated one at a time, reusing the same polynomial-size memory
All 2^m amplitudes of the state vector are stored simultaneously in polynomial space
A quantum circuit can be simulated in polynomial time, so it trivially fits in polynomial space