Directions: In the question below, the first and the last (correctly ordered) sentences are numbered 1 and 6, respectively. The rest of the passage is split into four parts and labelled B, C, D and E. These four parts are not given in a proper order. Choose the correct combination from the given alternatives.

1. The atmosphere of Earth serves as a key factor in sustaining the planetary ecosystem. B. Dry air consists of 78% nitrogen, 21% oxygen, 1% argon and other inert gases, such as carbon dioxide. C. The remaining gases are often referred to as trace gases, among which are the greenhouse gases such as water vapour, carbon dioxide, methane, nitrous oxide and ozone. D. The thin layer of gases that envelops Earth is held in place by the planet's gravity and is called air. E. Filtered air includes trace amounts of many other chemical compounds.
2. Air also contains a variable amount of water vapour and suspensions of water droplets and ice crystals seen as clouds.