= Solution
The helium yield is primarily neutron-limited, so small changes in <cosmological baryon density> have only a weak effect compared with their effect on residual deuterium. Increasing the baryon-to-photon ratio makes deuterium destruction less effective relative to its formation and releases the <deuterium bottleneck> at a slightly higher temperature. Nuclear reactions also proceed faster. Fewer neutrons decay before capture, so \b[a small increase in baryon density slightly increases helium-4; a decrease slightly lowers it.]
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