Instruct: Identify the main and secondary category of Arxiv papers based on the titles
Query: The 21-cm Background from the Cosmic Dark Ages: Minihalos and the
  Intergalactic Medium before Reionization
16
The categories of this paper are: stat
11
The categories of this paper are: hep-lat
24
The categories of this paper are: dg-ga
22
The categories of this paper are: chao-dyn
17
The categories of this paper are: q-fin
35
The categories of this paper are: mtrl-th
27
The categories of this paper are: comp-gas
28
The categories of this paper are: alg-geom
4
The categories of this paper are: cond-mat
32
The categories of this paper are: atom-ph
33
The categories of this paper are: plasm-ph
8
The categories of this paper are: hep-ex
23
The categories of this paper are: solv-int
13
The categories of this paper are: quant-ph
21
The categories of this paper are: adap-org
31
The categories of this paper are: ao-sci