下列说法正确的是（ ）
A．物体静止在水平桌面上，物体对桌面的压力就是物体受到的重力
B．形状规则的物体的重心一定在其几何中心上
C．在弹性限度的范围内，弹性体的弹力跟弹性体的长度成正比
D．用一根细竹竿拨动水中的木头，木头受到竹竿的推力，是由于竹竿发生形变而产生的
A．物体静止在水平桌面上，物体对桌面的压力大小等于重力，压力与重力性质不同，故A错误；
B．重心不一定在物体的几何中心上，只有质量分布均匀，形状规则的物体，重心才在其几何重心，故B错误；
C、根据弹簧弹力的表达式F=kx，x为伸长量或压缩量，k为弹簧的劲度系数，可知：x是伸长量或压缩量，而不长度，故C错误；
D、木头受到竹杆的弹力，是由于竹杆发生形变而产生的，不是木头发生形变产生的．故D正确．
故选D
| The blue of the sea is caused by the scattering of sunlight by tiny particles suspended in the water. Blue light, being of short wavelength, is scattered more efficiently than light of longer wavelengths. Although waters of the open ocean are commonly some shade of blue, green water is commonly seen near coasts, especially in tropical or subtropical regions. This is caused by yellow pigments being mixed with blue water. Phytoplankton are one source of the yellow pigment. Other microscopic plants may color the water brown or brownish-red. Near the shore, silt or sediment in suspension can give water a brownish hue. Outflow of large rivers can often be observed many miles offshore by the coloration of suspended soil particles. Marine phytoplankton (Greek for "plant wanderers") are microscopic single-celled plants that include diatoms, dinoflagellates, coccolithophorids, green algae, and blue-green algae, among others. The growth of these organisms, which photosynthesize light, depends on a delicate balance of nutrient enrichment via vertical mixing, which is often limited by the availability of nitrogen and light. Diatoms are one-celled plants with patterned glass coverings. Each glass, or silicon dioxide box, is ornamented with species-specific designs, pits, and perforations making them popular with microscopists and, more recently, electron scanning microscopists. |
The main idea of this passage is that ______.
A．light causes sea color
B．sea coloration is varied because of a combination of length of light waves and microscopic plant life and silt
C．microscopic plant life causes sea color
D．water composition causes sea color