单选题Apparent wind speed blowing across a ship under tow can be measured by a(n)（）.A barometerB wind vaneC anemometerD thermometer

29． Why are green plants and grass helpful?

A． Because they can hold the water．

B． Because they can stop the wind from blowing the earth away．

C． Because they can keep the earth wet．

D．All the above．

A survival craft being used to pick up a person who has fallen overboard from a ship should approach the person ______.

A.at a high rate of speed

B.under oars

C.against the wind

D.with the wind

A.blowing

B.is blowing

C.am blowing

D.are blowing

Your radar is set on a true motion display．Which of the following will NOT appear to move across the PPI scope ________．

A.Echoes from a buoy

B.Own ship's marker

C.Echo from a ship on the same course at the same speed

D.Echo from a ship on a reciprocal course at the same speed

When recording the wind direction in the weather log，you would report the ______．

A.direction the wind is blowing toward

B.direction the wind is blowing from

C.duration of the maximum gust of wind

D.wind chill factor

A semisubmersible rig under tow should be ballasted down if ______.

A.motion begins to be excessive

B.water begins to break over the pontoons

C.period of roll exceeds 12 seconds

D.forward progress is halted by head wind

If the current and wind are in opposite directions，the sea surface represents ______.

A.a greatly reduced wind speed

B.a higher wind speed than what really exists

C.a lower wind speed than what really exists

D.more turbulent winds

Propeller pitch speed minus ship speed divided by the propeller pitch speed is termed ___.

A.apparent slip

B.true slip

C.pitch

D.propulsive efficiency

The velocity of the apparent wind can be more than the true wind，and come from the same direction，if certain conditions are present．One condition is that the ______．

A.Ship's speed must be less than the true wind velocity

B.True wind must be from dead astern

C.True wind velocity must be faster than the ship's speed

D.True wind must be from dead ahead

A wind vane on a moving vessel shows ______.

A.dead reckoning wind direction

B.true wind direction

C.apparent wind direction

D.estimated wind direction