Riddle

There are millions of me. I am very small but when moving fast I am deadly.

Choose an answer

A.Bullets
B.Dust
C.Hailstones
D.Sand
Hint: It's a word with 4 letters, starting with "S".

Answer

Sand

Why this is the answer

Individual grains of sand are tiny and harmless, but wind-driven sand moving fast enough, as in a sandstorm, can be dangerous.

This riddle is filed under Hard/Difficult Riddles and rated Medium.