Instruct: Identify the main and secondary category of Arxiv papers based on the titles
Query: Ultra High Energy Cosmic Rays: the present position and the need for
  mass composition measurements
18
The categories of this paper are: econ
35
The categories of this paper are: mtrl-th
12
The categories of this paper are: nucl-th
13
The categories of this paper are: quant-ph
31
The categories of this paper are: ao-sci
28
The categories of this paper are: alg-geom
22
The categories of this paper are: chao-dyn
8
The categories of this paper are: hep-ex
19
The categories of this paper are: eess
0
The categories of this paper are: hep-ph
24
The categories of this paper are: dg-ga
16
The categories of this paper are: stat
32
The categories of this paper are: atom-ph
37
The categories of this paper are: supr-con
27
The categories of this paper are: comp-gas
10
The categories of this paper are: q-bio