In verb conjugations, beginning students are typically taught only a single usage for a verb conjugation. Base-II (or ます form) conjugation, or the い conjugation, is commonly taught as the form that is used for the ます form of verbs. While this is true, the ます form is not the only usage that it has.
One of the more common usages for base-II conjugation is for combining or linking verbs and phrases. Usually this is taught as one of the functions of the base-て conjugation, as this form of verb linking is most common, but it can also be done with base-II conjugation. To link verbs using base-II conjugation simply:
- Conjugate the verb to the base-II (or ます) form.
- Use the next verb or phrase.
Examples:
食べ変える (to switch food items (to eat)) --see note
歩き続ける (to continue walking)
友達を呼び、外食に行く (call on a friend and go out to eat)
The difference between using the ます form of the て form conjugation is mostly in the formality conveyed in the sentence. If you are talking to a superior, be it at work, school, or politics, you would do well to use the ます form to link the phrases, as it is more formal or literary.
Note:
It should be noted that sometimes, when liking two verbs directly like this, the result can be someone unpredictable. There is a plethora of verbs that, though combined in this way, do not have the expected meaning. For example:
行く (to go) + 成る (to become) = 行き成り (abrupt; without warning)
This was not asked in the question, but I think that it should be addressed.
With regard to correction A:
あなたがそうしないなら彼らは鳴きやみません。
While grammatically correct, I think that there is a nuance that needs to be addressed. Using なら as an 'if' typically carries the feeling of, "If it is true that ____, then ____." What comes before なら in this case is usually assumed or hypothetical.
What you are talking about here isn't hypothetical, it's more certain. In this case I would use と as your 'if.' It carries a more certain "If ____, then ____" relationship.
あなたがそうしないと彼らは鳴きやみません。
