Questions 63－68 are based on the following passage.

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.

63. Green water near coastlines is almost always caused by_____.

A. sand color

B. red pigments in coastal waters

C. blue pigment

D. reflected light and yellow pigment from plant life

64. Phytoplankton are the source of which color pigment?

A. Red.

B. Green.

C. Yellow.

D. Blue.

65. What can give waters a brownish hue near the shore?

A. Sediment.

B. Phytoplankton.

C. Blue pigment.

D. Diatoms.

66. Which of the following is not a type of phytoplankton?

A. Green algae.

B. Diatoms.

C. Blue-green algae.

D. Amoeba.

67. The growth of phytoplankton is often limited by the availability of_____.

A. oxygen

B. hydrogen

C. nitrogen

D. carbon dioxide

68. 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