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Inventories
3 Months Ended
Mar. 31, 2021
Inventory Disclosure [Abstract]  
Inventories
4.Inventories

Inventories as of December 31, 2020 and March 31, 2021 consisted of the following:


   December 31,   March 31, 
   2020   2021 
Raw materials  $757,857   $1,042,448 
Work in progress   2,338,342    1,911,177 
Finished goods   2,156,646    3,545,688 
   $5,252,845   $6,499,313 

During the three months ended March 31, 2020 and 2021, write-downs of obsolete inventories to lower of cost or market of $409,062 and $233,305, respectively, were charged to cost of revenues.