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问题 选择题

如图所示，质量为m的工件置于水平放置的钢板C上，二者间的动摩擦因数为μ，由于光滑导槽A、B的控制，工件只能沿水平导槽运动，现在使钢板以速度v1向右运动，同时用力F拉动工件（F方向与导槽平行）使其以速度v2沿导槽运动，则:

A.钢板对工件的摩擦力不等于μmg

B.拉工件的力F小于μmg

C.工件与导槽A之间没有弹力

D.工件与导槽B之间没有弹力

答案

BC

题目分析：根据题意可得：工件有相对于钢板水平向左的速度和沿导槽的速度，根据运动的合成与分解可得：故工件相对于钢板的速度如图所示，

滑动摩擦力方向与相对运动方向相反，所以有：

因此F的大小为；故A错误，B正确，工件有相对于钢板水平向左的速度，则工件与A没有弹力，故C正确，D错误；

单项选择题

男性，48岁，反复出现游走性四肢红斑结节，曾消退。近日再发，为求诊断活检显示皮下呈间隔性脂膜炎，可见多核巨细胞反应和肉芽肿形成，小静脉至中等大静脉炎，诊断是（）

A.结节性红斑

B.硬结性红斑

C.表浅静脉炎

D.皮肤血管炎

E.以上均不是
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单项选择题

Young girls at high risk for depression appear to have a malfunctioning reward system in their brains, a new study suggests. The finding comes from research that (1) a high-risk group of 13 girls, aged 10 to 14, who were not depressed but had mothers who (2) recurrent depression and a low-risk group of 13 girls with no (3) or family history of depression. Both groups were given MRI brain (4) while completing a task that could (5) either reward or punishment.

(6) with girls in the low-risk group, those in the high-risk group had (7) neural responses during both anticipation and receipt of the reward. (8) , the high-risk girls showed no (9) in an area of the brain called the dorsal anterior cingulated cortex (背侧前扣带皮质), believed to play a role in (10) past experiences to assist learning.

The high-risk girls did have greater activation of this brain area (11) receiving punishment, compared with the other girls. The researchers said that this suggests that high-risk girls have easier time (12) information about loss and punishment than information about reward and pleasure.

"Considered together with reduced activation in the striatal (纹状体的) areas commonly observed (13) reward, it seems that the reward-processing system is critically (14) in daughters who are at elevated risk for depression, (15) they have not yet experienced a depressive (16) ," wrote Ian H. Gotlib, of Stanford University, and his colleagues. " (17) , longitudinal studies are needed to determine whether the anomalous activations (18) in this study during the processing of (19) and losses are associated with the (20) onset of depression," they concluded. The study was published in the April of the Archives of General Psychiatry.

13（）

A.after

B.among

C.with

D.during
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