= Solution
For a moderately faster <cosmic expansion> that still permits efficient nuclear burning, \b[the hydrogen mass fraction is smaller], since cooling from <cosmological weak freeze-out> to helium formation takes less time, fewer free <neutrons> decay into <protons>, and more surviving neutrons enter <helium-4>. If the change also affects <cosmological weak freeze-out>, its higher freeze-out <temperature> leaves a larger initial <neutron-to-proton ratio> and reinforces the helium increase. The conclusion assumes efficient <neutron-limited helium synthesis>: an unspecified arbitrarily rapid expansion could instead outrun the <nuclear reactions>, so the <nuclear-burning qualification of the helium expansion-rate response> prevents an unconditional assertion.
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