People ----how light behaves thousands of years ago.
A
must have begun observing
B
must began to observe
C
must to being to observe
D
none of thses
Correct Answer: must have begun observing
Modal auxiliary - এর পরে verb - এর base form বসে।এদিক দিয়ে (a)ই একমাত্র সঠিক answer অন্যগুলো ভুল। Must + perfective হলো সঠিক answer। Must + have v.p.p হলো must + Perfective এর সঠিক রুপ।