In one kind of abnormal chromosome inheritance called down syndrome, a child has three copies of
a. chromosome x.
b. chromosome 21.
c. chromosome y.
d. chromosome 23.

answer : haversian

[explanation: -(compact bone is organized as parallel columns, known as haversian systems, which run lengthwise down the axis of long bones. these columns are composed of lamellae, concentric rings of bone, surrounding a central channel, or haversian canal, that contains the nerves, blood vessels, and lymphatic system of the bone)]

d. changes in climate and historical records

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