Why is the technique of measuring geometric parallax only effective for measuring distances that are less than a few hundred parsecs?

Geometric parallax measures a star’s apparent shift against distant background stars as Earth moves around the Sun. The farther away the star is, the smaller that apparent shift becomes, making it difficult to distinguish from measurement limits at large distances. So stars more than a few hundred parsecs away show too little apparent motion for this method to measure reliably. The correct choice is B, not A: distant stars can be observed, but their parallax is extremely small.

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