🔍 Key Concepts

• Crystal detector function in early radio receivers (detecting RF signals) • Typical semiconductor minerals used historically in crystal radios • Difference between natural crystals (like galena, iron pyrites, carborundum) and refined semiconductor materials (like silicon)

💭 Think About

• Think about which materials were actually used in early crystal radio sets as the rectifying element to detect radio signals. • Ask yourself: are these materials all capable of acting as a semiconductor junction or rectifier? • Consider whether any one of the options would exclude a substance that you know was commonly used as a detector material.

✅ Before You Answer

• Verify which of the listed substances are known semiconductor or rectifying crystals. • Check if each pair listed in A, B, and C includes only materials historically used as crystal detectors. • Before choosing D, make sure every material named in A, B, and C fits the definition of a usable crystal detector material.