Comma after introductory words, phrases, clauses.
Answer: Sentence C is correct. If you omit the first word, the sentence means exactly the same thing. Well is an introductory word that a comma should separate from the rest of the sentence. In sentence A, there is no comma after well.
Introductory Phrases In most cases, introductory phrases perform important functions: they are used to introduce something. Simply speaking, they build a stage for the rest of the sentence. Putting an introductory element to the sentence, one needs to use a comma after it to let the reader know that the key subject of the sentence is yet to follow.
Separating the parts of a sentence Commas are used to separate an introductory word or phrase from the main sentence, or to separate a word or phrase that briefly interrupts the flow of the sentence. In the examples below, the introductory and interrupting words or phrases have been italicised. Nevertheless, many critics see value in this theory.
The traditional rule is to use commas to separate introductory elements from the main sentence. For example, writing guides usually say to use a comma after every introductory element—whether word, phrase or clause—to clarify where it ends and the rest of the sentence begins.
Commas are placed after the introductory clause, phase or words. If there is more than one introductory clause, phase or words a comma is placed after each introductory clause, phase or words. Examples: Wow, that was a great movie. While I got dressed, The radio was playing a great song. Excellent.
Put a comma before a coordinating conjunction (for, and, nor, but, or, yet, so) that separates two independent clauses. Put a comma after introductory words, phrases, or clauses in a sentence. Use commas to set off elements that interrupt or add information in a sentence. Use commas to visually separate distinct but related items.
Missing comma after an introductory element Check your sentences to see which ones open with an introductory word, phrase, or clause. Readers usually need a small pause between the introductory element and the main part of the sentence, a pause most often signaled by a comma.