Explanation:
The correct answer is option 'A' - Time of the first bell in the school.

Time intervals:
Time intervals refer to a specific duration or period of time. They measure the amount of time between two points or events.

Options:

a) Time of the first bell in the school:
This option does not express a time interval. It refers to a specific point in time when the first bell rings in the school. It does not indicate a duration or a period of time. Therefore, it is not an example of a time interval.

b) A day:
A day represents a time interval. It refers to a period of 24 hours, starting from midnight and ending at the next midnight. It is a commonly used unit of time to measure the duration of various activities or events.

c) A second:
A second is another example of a time interval. It is a unit of time measurement in the International System of Units (SI). It represents a duration of 1/60th of a minute or 1/3,600th of an hour. It is commonly used to measure short durations or intervals.

d) A school period:
A school period is also a time interval. It refers to a specific duration of time allocated for a particular subject or activity in a school. It can vary in length depending on the school's schedule and may range from a few minutes to an hour.

Conclusion:
Out of the given options, the only one that does not express a time interval is option 'A' - Time of the first bell in the school. This option refers to a specific point in time rather than a duration or period of time.