Questions

It is a Postbox or a Mailbox which contains thousands of letters or mails or messages.

Coal is chemically complex fuel. Whenever it is burned, gases are given off and particles of ash, called "fly ash," are released. Release of sulfur from burning into the air. The sulfur in coal combines with oxygen to form sulfur dioxide, which can be a major source of air pollution if emitted in large enough quantities. This stuff comes back as acid rain when it attaches itself to a raindrop. 

Today, many of the effects of coal burning have been reduced significantly or eliminated. This is super-nasty and is only stopped by buying low-sulfur coal from Montana or Wyoming. The eastern power plants have to pay to move the coal from out west and this increases the price of power production.

American startup Planet Labs, that launches small satellites into orbit and sells the imagery, has announced its acquisition of Google's 'Terra Bella' satellite business. As part of the agreement, 60 Google employees will join the startup founded by ex-NASA scientists. Terra Bella, was acquired by Google in 2014 for $500 million and that time it was known as Skybox Imaging.

A cold or flu illness.

The Great Depression was a severe worldwide economic depression that took place mostly during the 1930s, beginning in the U.S. The timing of the Great Depression varied across nations; in most countries it started in 1929 and lasted until the late-1930s. It was the longest, deepest, and most widespread depression of the 20th century. In the 21st century, the Great Depression is commonly used as an example of how far the world's economy can decline.