✔ 最佳答案
In 712escd ; the diode is placed in a reversed direction. [Diode is a polar element; in order to light it, current must flow in from anode.]
In a231scd current flow through the LED is limited by the high resistance voltmeter, current flow through is so small that it is very dim.
In order to make it works properly, the high resistance voltmeter should be connected parallel to the LED.