If a boat can have a speed of 4 km/hr in still water, for what values of speed of river flow, it can be managed to row boat right across the river, without any drift ?

Options

A. greater than or equal to 4 km/hr
B. greater than zero but less than 4 km/hr
C. only 4 km /hr
D. None of these

Explanation

To solve the problem, we need to determine the values of the speed of the river flow (Vr) that allow a boat to row directly across the river without any drift. The boat's speed in still water (Vb) is given as 4 km/hr.

### Step-by-Step Solution:

1. **Understanding Drift**:
- Drift occurs when the boat is carried downstream by the river's current while trying to row directly across. To avoid drift, the boat must be angled upstream.

2. **Setting Up the Problem**:
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