Which of the following is a definition of a CSS id?

Question

BlackTom AI · Accepted Answer

Start by restating what the question is asking: identifying which option defines a CSS id selector. 
Option 1: h2#BlueHead {font-weight: bold; color: blue;}
This is a valid CSS rule that uses an id selector. The syntax #BlueHead targets the element with id="BlueHead"; combining it directly with the element tag h2 is also correct and common. The declarations inside set font-weight to bold and color to blue, which is a proper usage.
Option 2: .RedLarge { font-size: 20pt; color: red;}
This rule uses a class selector, not an id selector. The leading dot . indicates a class name, so while the rule itself is valid CSS, it does not define or reference an id. It therefore does not answer the question about an id.
Option 3: p.id { font-family : Courier new;}
This selector is ambiguous for our goal. It reads as find p elements with a class named id (since .id would be a class), not an element with a specific id. If the intention were to select by id, the correct syntax would be p#idName or #idName alone. As written, it does not define an id selector either.
Option 4: All of these.
This statement would be true only if every previous option defined an id selector, which is not the case here since options 2 and 3 do not define an id selector. Therefore, this choice is incorrect.
To summarize the distinctions: the id selector uses a hash symbol followed by the id value (e.g., #BlueHead) and can be used alone or appended to an element name (e.g., h2#BlueHead); class selectors use a leading dot; and the combination in option 3 does not express an id selector properly. The only option that directly defines an id selector is option 1, making it the correct choice.

Which of the following is a definition of a CSS id?单项选择题

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