Table:
class	number (s)	quantity	year (s) of manufacture	type

d i	names	15	1871 - 1875	b n2t

d ii (old)	names	4	1873	b n2t

d ii	2400 - 2472	73	1898 - 1904	c n2t

d iii	names	6	1873	b n2t

d iv	names , 1701 - 1737	132	1875 - 1897	b n2t

d v	names	10	1877 - 1878	c n2t

d ix	names , 1931 - 1960 , 2101 - 2115	55	1888 - 1899	1b n2t

d xii	2201 - 2296	96	1897 - 1904	1′b2′ n2t

pt 2 / 3	6001 - 6097	97	1909 - 1916	1b h2t

pt 2 / 4 n	6501 - 6502	2	1909	2′b n2t

pt 2 / 4 h	5001 - 5012	12	1906 - 1909	1′b1′ h2t

pt 2 / 5 n	5202 - 5210	9	1907	1′b2′ n2t

pt 2 / 5 h	5201	1	1906	1′b2′ h2t

pt 3 / 6	6101 - 6110	10	1923	1′c2′ h2t

gt 2x4 / 4	5751 - 5765	15	1913 - 1914	d′d h4vt

gt 2x4 / 4	5766 - 5775	10	1922 - 1923	d′d h4vt

r 3 / 3	2473 - 2490	18	1906 - 1913	c n2t

r 3 / 3	4701 - 4790	90	1921 - 1922	c n2t

r 4 / 4	4151 - 4183	33	1918 - 1919	d n2t

r 4 / 4	4184 - 4192	9	1924 - 1925	d n2t

Based on the provided table, please extract the cells' values located at the specified positions, as denoted by row and column indices. Return each cell value as JSON in the format {"row_id":"m", "column_id":"n", "cell_value":"<Corresponding Cell Value>"}.
Positions:
the 12th row and the 3rd column
the 2nd row and the 2nd column
the 5th row and the 1st column
the 18th row and the 1st column
the 5th row and the 3rd column
the 21th row and the 4th column

The target cell values are as follows:
{"row_id":"12", "column_id":"3", "cell_value":"12"}
{"row_id":"2", "column_id":"2", "cell_value":"names"}
{"row_id":"5", "column_id":"1", "cell_value":"d iii"}
{"row_id":"18", "column_id":"1", "cell_value":"r 3 / 3"}
{"row_id":"5", "column_id":"3", "cell_value":"6"}
{"row_id":"21", "column_id":"4", "cell_value":"1924 - 1925"}