Detrital grains of are likely to be concentrated in placer deposits of california

Detrital grains of native gold are likely to be concentrated in "placer deposits of California".

Explanation:

The "Sierra Nevada mountains" in California host one of the largest placer deposits of native gold. These deposits were discovered in the mid-nineteenth century resulting in a gold rush and giving the region enormous popularity. The Sierra Nevada mountains are also known for its rich mineral wealth of silver, tungsten, granite, gypsum, calcite, quartz, feldspar, mica, and dolomite.

The answer is a but it also might be 7 so

81 is a composite number