Yes, but the subordinate clause in C also has a subject and a verb. The subject is "doing so" and the verb phrase is "might be." It's a similar construction to "Submitting your application early might be helpful." Should I submit my application early? Sure. Doing so might be helpful.
The problem in C is one of meaning: making wine is not poisonous. Also, the word "themselves" is not necessary in C, since no one is doing something to themself (as in D).
The problem in C is one of meaning: making wine is not poisonous. Also, the word "themselves" is not necessary in C, since no one is doing something to themself (as in D).
