Which of the following distance-time graphs shows a truck moving with speed which is not constant?

Options

A.
B.
C.
D.

Explanation

To determine which distance-time graph shows a truck moving with a speed that is not constant, we can follow these steps:

### Step-by-Step Solution:

1. **Understand Speed and Distance-Time Graphs**:
- Speed is defined as the distance traveled per unit of time. Mathematically, it is expressed as:
\[ \text{Speed} = \frac{\text{Distance}}{\text{Time}} \]
- In a distance-time graph, if the speed is constant, the graph will be a straight line. This is because distance increases proportionally with time.

2. **Identify Characteristics of Constant Speed**:
- If the speed is constant, the graph will show a straight line. This indicates that for every unit of time, the distance increases by a fixed amount.

3. **Identify Characteristics of Non-Constant Speed**:
- If the speed is not constant, the graph will not be a straight line. Instead, it may curve or change slope, indicating that the distance is not increasing uniformly with time.

4. **Analyze the Given Options**:
- **Option 1**: Straight line graph - indicates constant speed.
- **Option 2**: Straight line graph - indicates constant speed.
- **Option 3**: Parabolic graph - indicates changing speed (not constant).
- **Option 4**: Straight line graph - indicates constant speed.

5. **Select the Correct Answer**:
- From the analysis, options 1, 2, and 4 represent constant speed (straight lines). Only option 3, which is a parabolic graph, indicates that the speed is not constant. Therefore, the correct answer is:
\[ \text{Option 3} \]

### Summary of the Solution:
The distance-time graph that shows a truck moving with speed that is not constant is represented by a parabolic graph, which corresponds to option 3.