Microprocessor
The output data lines of microprocessor and memories are usually tristated because

 It increases the speed of data transfer over the data bus
 More than one device can transmit information over the data bus by enabling only one device at a time
 More than one device can transmit over the data bus at the same time
 The data line can be multiplexed for both input and output

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Microprocessor
When RET instruction is executed by any subroutine then

 the top of the stack will be popped out and assigned to the PC.
 without any operation, the calling program would resume from instruction immediately following the call instruction and also the PC will be incremented after the execution of the instruction.
 the PC will be incremented after the execution of the instruction.
 without any operation, the calling program would resume from instruction immediately following the call instruction.

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