Question

Which of the below-given options is/are not the unit of time?

• A. Second
• B. Parsec
• C. Year
• D. Light year

Answer 1

Answer: (B), (D)

Light year

Step 1: Explanation of the correct options

In the case of option B,

1. A parsec is a unit of length used to calculate the large distances between astronomical objects which are outside the solar system.
2. $1\mathrm{parsec}=3.26\mathrm{light}\mathrm{year}$
3. Thus, a parsec is not the unit of time.

In the case of option D,

1. A light-year is a distance traveled by the light in a vacuum in one year.
2. It is used in astronomy to measure the distances of nearby stars.
3. $1\mathrm{light}-\mathrm{year}=9.467\times {10}^{15}\mathrm{m}$

Step 2: Explanation for the incorrect options

In the case of option A,

1. Second is the standard international unit of time.
2. $1\mathrm{second}=\frac{1}{3600}\mathrm{hour}$

In the case of option C,

1. A year is approximately $365\mathrm{days}$ long although leap year is an exception.
2. A year is the time taken by the Earth to complete one complete orbit around the sun.
3. A normal year is actually $365.25$ days long, but a calendar has $365.25$ days.

Hence, options B and D are correct options.