An airplane took off $40 \text{ minutes}$ later than its scheduled time for a destination $1600 \text{ km}$ away. To reach on time, the pilot increased the speed by $400 \text{ km/h}$. What was the normal speed of the airplane?

Options:

• A: $600 \text{ km/h}$
• B: $800 \text{ km/h}$
• C: $1000 \text{ km/h}$
• D: $1200 \text{ km/h}$

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