You will be given a trivia clue, and the category it belongs to. You should answer with the best answer that belongs in the category and is described by the clue. For simplicity, answers should be in all lower cased letters.
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Question: Category: PARTICLE PHYSICS 
Clue: To get an element's mass number, add its protons & these

Answer: neutrons


Question: Category: BUGGY 
Clue: Salticidae, the jumping types of these arachnids, have hairy bodies & feed on insects & on others of their kind

Answer: spiders


Question: Category: STUPID ANSWERS 
Clue: Number of different basic shapes in a box of Post Alpha-Bits

Answer:
26