下列有关对基因的理解，不正确项是（ ） A．基因是遗传物质中决定生物性状的最小单

如图所示为火车站用来装卸煤炭使用的水平传送带模型，水平传送带的长度为L=8m，传送带的皮带轮半径均为R=0.4m，皮带轮转动的角速度为ω=10rad/s．传送带的底部距地面的高度为h=4.2m，现有一块矩形煤炭（视为质点）无初速度地释放在水平传送带的A点位置处（A、B分别为皮带轮中心轴正上方的两个点）．已知煤炭与皮带之间的动摩擦因数为μ=0.2．皮带轮与皮带之间始终不打滑．空气阻力不计，g取10m/s²．回答下列问题：

（1）水平传送带的传动速度为多少？该块煤炭从释放到和传送带共速时，煤块和传送带运动的距离分别是多少？

（2）该块煤炭到达B点后是沿着皮带轮的圆弧滑下，还是离开圆弧飞出？说明原因（要求有必要的理论运算）．

（3）该块煤炭落在水平地面上的落点与O₂点正下方的水平距离多大？

以下哪种疾病会出现外周血网织红细胞绝对值减少

A．缺铁性贫血

B．巨幼细胞性贫血

C．再生障碍性贫血

D．溶血性贫血

E．慢性失血性贫血

巷道风流划分中，无支架的巷道应划分为距顶、帮、底各200mm的巷道空间的风流。

简述产品质量特性内容？

TEXT B
One of the mysteries of nature is animal communication. Creatures of the sea move through their liquid element without any hearable sound. Yet they are able to sense the silent passing or attack of others of their kind. This is because bodies moving in water create a pressure wave, something like a gust of wind produced by a passing automobile. A narrow sensor system running along each side of the shark, from eye to tail, receives and interprets these pressure waves. I have seen sharks appear swiftly, obviously drawn by the motion of a diver’s hands, which had sent out p waves.
One might think it impossible to distinguish odors in water. Yet sharks, surprisingly, are able to follow a scent across miles of ocean and arrive at its exact source. A diver who spears a fish and attaches its bloody body to his belt, and then goes on with the hunt, becomes a natural prey to the shark. I have seen sharks follow a scent exactly like the dogs of a hunting pack. It is no wonder that the Greeks gave them the name "hounds of the sea."
One of the most widely believed--and dangerous--legends about the shark is that he has poor eyesight. On the contrary, the shark is well equipped to see at a distance and to distinguish among forms. This fact was proved to me one day when I went into the water off the coast of Africa. I sighted a shark at some distance from me as I was floating at a very shallow depth. Since I was making no movement, the sound of bubbles from my diving apparatus would be confused with the light splashing of the water on a rock. I turned my eyes away for a few seconds, to study the design of a giant ray just beneath me. I am not sure now whether it was simple instinct or a feeling of movement, but I turned back abruptly toward the location of the shark. And, immediately, every muscle of my body tensed. He was no more than 30 feet away, launched toward me as hard and swift as a missile.
The sight of a shark coming at you head-on is strange. Obviously it is from this angle that he seems most frightening--the very symbol of evil, with the half-opened mouth and the three regularly spaced fins (鳍). When the shark had approached to within two feet of the rubber fins I had thrown at him as a gesture of self-defense, he turned suddenly and swam back toward the depths. There had been no sound, no scent. It appears certain that sight alone was responsible for this approach.

The author escaped from the attack of the shark by ______.

A．turning around suddenly

B．screaming

C．swimming back to the shore

D．throwing rubber fins at him