PERFECT is used, i.e., 'had RESIGNED'Thus, B is wrong.The sentence should be 'It is difficult to explain why Subhas Bose had resigned from the National Congress.' There are four kinds of sentences:Declarative sentence: A declarative sentence simply makes a statement or expresses an opinion. In other words, it makes a declaration. This kind of sentence ends with a PERIOD.Example: The school shall remain closed tomorrow. Imperative sentence: An imperative sentence gives a command or makes a REQUEST. It USUALLY ends with a period but can, under certain circumstances, end with an exclamation point.Example: Please shut the door.Interrogative sentence: An interrogative sentence asks a question. This type of sentence often begins with who, what, where, when, why, how, or do, and it ends with a question mark.Example: Why are you late?Exclamatory sentence: An exclamatory sentence is a sentence that expresses great emotion such as excitement, surprise, happiness and anger, and ends with an exclamation point.Example: Hurrah! We won the match.