Which of the following is not a unit of time
A) par - sec `" "` B) light - year `" "` C) micron `" "` D) sec

Options

A. Only A
B. A and B
C. A, B & C
D. A, B , C , D

Answer

To determine which of the following options is not a unit of time, we will analyze each option provided:

**Options:**
A) par - sec
B) light - year
C) micron
D) sec

**Step 1: Analyze each option**

1. **Option A: par - sec**
- "Per second" (often abbreviated as "sec") is a unit of time. It is commonly used in various scientific contexts, especially in physics and engineering to denote rates or frequencies. Therefore, this option is a unit of time.

2. **Option B: light - year**
- A light-year is a unit of distance, not time. It represents the distance that light travels in one year. The speed of light is approximately \(3 \times 10^8\) meters per second, and a light-year quantifies the distance light covers in that time frame. Thus, this option is not a unit of time.

3. **Option C: micron**
- A micron (or micrometer) is a unit of length, equivalent to \(10^{-6}\) meters. While it can be used in contexts involving time (like \(10^{-6}\) seconds), it is fundamentally a measure of length, not time. Therefore, this option is also not a unit of time.

4. **Option D: sec**
- "Sec" is the abbreviation for seconds, which is the standard unit of time in the International System of Units (SI). Thus, this option is a unit of time.

**Step 2: Conclusion**
- Based on the analysis, the options that are not units of time are **B) light - year** and **C) micron**. However, since the question asks for a single option that is not a unit of time, the most straightforward answer is **B) light - year**.

**Final Answer: B) light - year**

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