7. The speed of sound in a particular gas at a temperature 27C is 340 m/s. Then, the speed of sound in the same gas at a temperature 90C is: (a) 340 m/s (b) 357 m/s (c) 374 m/s (d) 411.4 m/s 8. 9. If temperature of a liquid is increased, choose the correct option regarding change in its surface tension and viscosity. (a) Surface tension decreases while viscosity increases (b) Surface tension increases while viscosity decreases (c) Both increases (d) Both decreases Two trains are approaching each other on the same track with velocity 100 km/h and 50 km/h. When they are at a distance of 300 km from each other, a bird starts flying from train 1 towards train 2. When the bird reaches train 2, it instantly reverses its direction and flies again towards train 1. This process continues till the bird is trapped between the trains when they collide. If the bird always travels at a constant speed 200 km/h, the total distance the bird flies is: (a) 200 km (b) 400 km (c) 300 km (d) 600 km