1. A method performed by data processing apparatus, the method comprising: receiving a query from a user device; receiving data identifying a plurality of resources determined to be responsive to the query; accessing an index of answer box topics, the index storing associations between resources and answer box topics; based on the accessing, analyzing the plurality of resources to identify a subset of resources of the plurality of resources that are annotated with a respective answer box topic, the answer box topic for each resource of the subset of resources based on content of the respective resource; determining a set of answer box topics from the respective answer box topics associated with the subset of resources; analyzing each answer box topic of the set of answer box topics to identify a first answer box topic that is i) responsive to the query and ii) based on the content of the respective resource the first answer box topic annotates; obtaining an answer box precursor for the first answer box topic using a mapping between a plurality of answer box topics and a plurality of answer box precursors, wherein the answer box precursor includes information that defines an answer box; and providing, to the user device, the answer box precursor for the first answer box topic in response to the query.
1. A method performed by data processing apparatus, the method comprising: receiving a query from a user device; receiving data identifying a plurality of resources determined to be responsive to the query; accessing an index of answer box topics, the index storing associations between resources and answer box topics; based on the accessing, analyzing the plurality of resources to identify a subset of resources of the plurality of resources that are annotated with a respective answer box topic, the answer box topic for each resource of the subset of resources based on content of the respective resource; determining a set of answer box topics from the respective answer box topics associated with the subset of resources; analyzing each answer box topic of the set of answer box topics to identify a first answer box topic that is i) responsive to the query and ii) based on the content of the respective resource the first answer box topic annotates; obtaining an answer box precursor for the first answer box topic using a mapping between a plurality of answer box topics and a plurality of answer box precursors, wherein the answer box precursor includes information that defines an answer box; and providing, to the user device, the answer box precursor for the first answer box topic in response to the query. 3. The method of claim 1 , further comprising: identifying, by the data processing apparatus, a second answer box topic that is i) responsive to the query and ii) based on the content of the respective resource the second answer box topic annotates, and wherein providing further comprises selecting, by the data processing apparatus, the first answer box topic as a consequence of a resource associated with the first answer box topic having a higher ranking relative to a reference associated with the second answer box topic.