21. Variables that can change or alter the testing system: A. can happen by chance only.
B. can happen by chance or by human activity.
C. can happen as a result of causes that are impossible to determine.
D. can happen by human activity only.

Name the bone/structure/process that fits the following description: a) a bone with a complex shape. b) found in the medullary cavity. c) a large, rough process. d) it is thin, like a plate. e) shaft of a long bone. f) a mature bone cell. g) a large, round, smooth process. h) it covers outer surface of bone. i) a shallow depression on surface of bone. j) a roughly cube-shaped bone. k) the covering of epiphysis. l) a very narrow ridge of bone. m) space between lamellae for an osteocyte. n) chamber within diaphysis o) also called wormian bone. p) it secretes new matrix. q) end of a long bone. r) a small, round hole. s) a matrix, arrangement in spongy bone. t) it is a chamber within a bone. u) a network allowing osteocytes to receive nutrients. v) one example is the patella. w) it houses artery, vein and nerve. x) almost flat, smooth surface of a bone. y) a narrow, pointed process.