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【简答题】
在单缝衍射中，当衍射角j满足【图片】时，单缝的波阵面可分为 个半波带
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【单选题】在电子商务条件下，买方应当承担的义务不包括（ ）
A.
买方应承担按照网络交易规定方式支付价款的义务
B.
准确、及时地进行电子资金的划拨义务
C.
买方应承担按照合同规定的时间、地点和方式接受标的物的义务
D.
买方应当承担对标的物验收的义务
【多选题】How do mitogens stimulate cell proliferation by inhibiting the Rb protein?
A.
As a general rule, mammalian cells will multiply only if they are stimulated to do so by extracellular signals, called mitogens, produced by other cells. If deprived of such signals, the cell cycle arrests in G 1 ; if the cell is deprived of mitogens for long enough, it will withdraw from the cell cycle and enter a nonproliferating state, in which the cell can remain for days or weeks, months, or even for the lifetime of the organism, as we discuss shortly.
B.
Escape from cell-cycle arrest—or from certain nonproliferating states—requires the accumulation of cyclins. Mitogens act by switching on cell signaling pathways that stimulate the synthesis of G 1 cyclins, G 1 /S cyclins, and other proteins involved in DNA synthesis and chromosome duplication. The buildup of these cyclins triggers a wave of G 1 /S-Cdk activity, which ultimately relieves the negative controls that otherwise block progression from G 1 to S phase.
C.
One crucial negative control is provided by the Retinoblastoma (Rb) protein. Rb was initially identified from studies of a rare childhood eye tumor called retinoblastoma, in which the Rb protein is missing or defective. Rb is abundant in the nuclei of all vertebrate cells, where it binds to particular transcription regulators and prevents them from turning on the genes required for cell proliferation.
D.
Mitogens release the Rb brake by triggering the activation of G 1 -Cdks and G 1 /S-Cdks. These complexes phosphorylate the Rb protein, altering its conformation so that it releases its bound transcription regulators, which are then free to activate the genes required for entry into S phase.
【单选题】在电子商务条件下,买方应当承担的义务不包括( )
A.
买方应承担按照网络交易规定方式支付价款的义务
B.
选完商品就立即就付款
C.
买方应承担按照合同规定的时间、地点和方式接受标的物的义务
D.
买方应当承担对标的物验收的义务
【单选题】下列关于轴力正负号的说法,正确的是( )。
A.
轴力指向横截面时为正，这样的轴力称为拉力
B.
轴力指向横截面时为正，这样的轴力称为压力
C.
轴力指向横截面时为负，这样的轴力称为拉力
D.
轴力指向横截面时为负，这样的轴力称为压力
【单选题】下列句子中,标点符号使用正确的一项是
A.
吴浩早上跟我说：“他脚崴了一下，今天不能来上课了。”
B.
他从报上看到某大学研究生院和《中华文化》编委会联合主办《中国文化与世界文化暑期讲习班》的招生启事，便立刻写信去报名。
C.
生长在人迹罕至的雪线以上的灵花异草，据说是稀世珍宝——一种很难求得的妇科良药。
D.
作文应做到“平字见奇，常字见险，陈字见新，朴字见色。”
【多选题】在电子商务条件下,买方应当承担的义务包括
A.
按照网络交易规定方式支付价款的义务
B.
按照合同规定的时间，地点和方式接受标的物的义务
C.
对标的物的质量承担担保义务
D.
对标的物验收的义务
【简答题】设函数F(x)为定义在凸集R上且具有连续二阶导数的函数,则,F(x)在R上为凸函数的充要条件是海森矩阵在R上处处 。
【单选题】下列关于轴力的说法中不正确的是（）。
A.
轴向拉（压）杆件横截面上的内力只有轴力
B.
轴力的方向与该截面的外法线方向一致的为正，相反的为负
C.
轴力的大小与杆件的截面形状、截面面积及尺寸大小无关
D.
杆件的轴力等于与其临近截面上的外力
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