61. Which river forms the Grand Canyon in the USA ______? 62. Which of these cloud forms is found at the highest altitudes ______? 63. Thickness of atmosphere around the Earth is ______? 64. Atmosphere around the Earth maintains its ______? 65. The layers of the atmosphere are divided into ______? 66. Above the Earth's surface, the troposphere extends to a height of ______? 67. Which next layer is above the troposphere ______? 68. The mesosphere is extended above the earth's surface upto ______? 69. Which one of the following phenomenon occurs as the water vapour condenses around a particle of smoke ______? 70. Lines joining places of the same earthquake intensity are called ______?