Choose the letter corresponding to the word to complete the paragraph.
In geology, a boulder is a_____fragment with size greater than 256 millimetres in diameter. Smaller pieces are called cobbles and pebbles. While a boulder may be small enough to move or roll manually, others are extremely ______. In common usage, a boulder is too large for a person to move. Smaller boulders are usually just called rocks or stones.
In places covered by ice sheets during _____ Ages, such as Scandinavia, northern North America, and Siberia, glacial erratics are common. Erratics are boulders picked up by ice sheets during their advance, and deposited when they melt. They are called "erratic" because they typically are of a different rock type than the bedrock on which they are deposited. One of them is used as the pedestal of the Bronze Horseman in Saint Petersburg, Russia.
Some noted rock _______involve giant boulders exposed by erosion, such as the Devil's Marbles in Australia's Northern Territory, the Horeke basalts in New Zealand, where an entire valley contains only boulders, and The Baths on the island of Virgin Gorda in the British Virgin Islands.
Boulder-sized clasts are found in some ______ rocks, such as coarse conglomerate and boulder clay. The climbing of large boulders is called bouldering.
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