21.如下圖所示,矯正線所作的彎折屬於:
(A)first order bend
(B)second order bend
(C)third order bend
(D)fourth order bend
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統計: A(122), B(598), C(88), D(3), E(0) #1845431
統計: A(122), B(598), C(88), D(3), E(0) #1845431