Some Unethical SEOs Have Given the Industry a Black Eye

Some unethical SEOs have given the industry a black eye through their overly aggressive marketing efforts and their attempts to manipulate search engine results in unfair ways. Suggested good practice(s) is (are) :

A) be wary of SEO firms and web consultants or agencies that send you email out of the blue
B) be wary of firms that guarantee a 1 ranking on Google
C) be wary if a company is secretive or won't clearly explain what they intend to do
D) all of the above
E) none of the above