The conventional rule is to take the plural form of the verb when two nouns are joined by the conjunction 'and'. However, there are exceptions. Only then can you break the rule.
When the two nouns refer to one thing only, then you can use the single form. For example: The president and the CEO joins the meeting today. In this example, the president and CEO apply to one person only. Hence, you use the singular form of the verb.
The word "cheap" in this sentence doesn't mean "not expensive" but rather "low-quality" or maybe "easy to break".
Glen Canyon Dam................
The correct answer would be : They are more likely to be reliable sources of information.
I hope that this helps you !
