Answer: D) white phosphorus Explanation: White phosphorus undergoes spontaneous combustion when exposed to air. In order to prevent this spontaneous ignition, it is preserved under water.

Answer: A) 25th and 21st Explanation: Number of students behind Manoj in rank = (31 - 7) = 24. So, Manoj is 25th from the bottom. Number of students behind Satya in rank = (31 - 11) = 20. So, Satya is 21st from the bottom.

Answer: D) All of the above Explanation: A computer has two types of memory. 1. Non-volatile and 2. Volatile.   Volatile memory is computer memory that requires power to maintain the stored information. * It is a temporary memeory which cannot contain information permanently. Example :: RAM or Random Access Memory   Non-volatile memory is computer memory that can retain the stored information even when not powered. * It is a permanent memory in which data is stored in permanently. Examples :: Hard disc, ROM, Floppy discs, Magnetic tape,...