The students in a certain physical education class are on either the football team or the basketball team, are on both these teams, or are not on either team. If 12 students are on the football team, 15 students are on the basketball team, 8 students are on both teams, and 9 students are not on either of these teams, how many students are in the class?

In the case of new crude oil tankers，the additional ballast permitted shall be carried in cargo tanks only if such tanks have been（）before departure from an oil unloading port or terminal.

Clouds that form as small white flakes or scaly globular masses covering either small or large portions of the sky are（）.

For the purpose of（）tank or hold washings from any vessel，an application shall be made to Harbor Authorities for approval.

The transfer procedures for oil products are required to be posted （）.

Those ship’s tanks that are particularly important for trimming the ship are the（）.

When preparing a ship for heavy weather，fuel oil day tanks should be（）.

The （） or balance tank collects the re-circulated fuel oil.

The （） or balance tank collects the re-circulated fuel oil.

Consonant sounds can be either （）or（）,while all vowel sounds are （）.

（） system is arranged to ensure that water can be drawn from any tank or the sea and discharged to any other tank or the sea as required to trim the vessel.

When you are transferring fuel oil to the setting tanks, precautions to be observed should include （）