position and movement

s=ut+(1/2)at2

if t=1,
s=0+(1/2)(10)(1)2
s=5 m

if t=2,
s=0+(1/2)(10)(2)2
s=20 m

if t=3,
s=0+(1/2)(10)(3)2
s=45 m
45 m係指該物件由t=0到t=3期間所行走的distance/displacement

0 s to 1 s is the first second
1.01 s to 2 s is the second second（第二秒）
2.01 s to 3 s is the third second

所以該物件於the third second行走的displacement
= Total displacement in the first three seconds - displacement in the first two seconds
= 45 m - (5+15) m
= 15 m

THRERFORE,
the answer is A.

2009-10-16 17:00:29 補充：
oh, sorry,
45-20=25
so, B is the correct anwer.

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