Questions

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

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.

Para-amino salicyclic acid (PAS) is used in the treatment of Tuberculosis.

Sad means being unhappy, sorrowful, mournful, wistful,...

Hence, Somber means cheerless i.e, unhappy which is synonym of sad. All other options are opposites of Sad.

Astigmatism is a common vision condition that causes blurred vision. It occurs when the cornea is irregularly shaped or sometimes because of the curvature of the lens inside the eye.

Integrated circuit (IC) is a tiny device that controls electric signals in such electronic equipments as computers and T.V sets. Many integrated circuits contain thousands of electronic parts on a paper - thin chip of silicon. These parts include capacitors, diodes, all in a very small, compact space.

The gas usually filled in the electric bulb is Nitrogen.