[ Comma ]
There is no reason to feel sad so try to smile.
A. There is no reason to feel sad so, try to smile.
B. There is no reason, to feel sad so try to smile.
C. There is no reason to feel sad, so try to smile.
D. No commas needed.

Random Topics:
Prepositions and Phrasal VerbsGerund or to InfinitivePassive Voice and Relative ClausesHow often - long - much - manyPast ModalsNounsGerund, Participle, Infinitive, or AppositiveConditional TensesThird ConditionalBe Going To vs. WillOther quiz:
Tenses › View‘Where is Kate?’ ‘She (watch) TV in the living room.’
A. watches
B. doesn’t watch
C. is watching
D. watch
Future Tenses › View
Oh, somebody is knocking! I … open it.
A. are going to
B. am going to
C. will
D. is going to