83. An electron tube was sealed off during manufacture at a pressure of

1.2×10^-7 mm of mercury at 27°C. Its volume is 100 cm³. The number of molecules that remain in the tube is

(a)  2×10¹⁶                                        (b) 3×10¹⁵

(c)  3.86×10¹¹                                  (d) 5×10¹¹

Answer-c