## 3 Method and Case Study
\begin{table}
\begin{tabular}{r l l} \hline \hline
**ID** & **URL** & **Subject** \\ \hline
11 bee & balloons-and-static-electricity/1.414/balloons-and-static-electricity\_es.html & Earth science \\
12 ben & beers-law-lab/1.418/beers-law-lab\_es.html & Physics \\
13 bla & bending-light/1.120/bending-light\_es.html & Physics \\
14 bui & blackbody-spectrum/1.07/blackbody-spectrum\_es.html & Earth science \\
15 bui & build-a-fraction/1.012/build-a-fraction,\_es.html & Math \\
16 cap & build-an-atom/1.614/build-an-atom,\_es.html & Physics \\
17 cha & capacitor-lab-basics/1.6.19/capacitor-lab-basics\_es.html & Physics \\
18 cir & charges-and-fields/1.0.47/charges-and-fields\_es.html & Physics \\
19 cir & circuit-construction-kit-dc/1.15/circuit-construction-kit-dc\_es.html & Physics \\
20 col & circuit-construction-kit-dc-virtual-lab/1.15/circuit-construction-kit-dc-virtual-lab\_es.html & Physics \\
21 con & color-vision/1.1.23/color-vision\_es.html & Biology \\
22 cou & concentration/1.3.20/concontration\_es.html & Physics \\
23 cur & curve-fitting/1.0.0/curve-fitting\_es.html & Math \\
24 dif & coulombs-law/1.0.9/coulombs-law\_es.html & Physics \\
25 pene & diffusion/1.0/4/diffusion\_es.html & Earth science \\
26 ene & energy-forms-and-changes/1.011/energy-forms-and-changes\_es.html & Physics \\
27 equ & energy-skate-park-basics/1.1.19/energy-skate-park-basics\_es.html & Physics \\
28 equ & equality-explorer/1.0.12/equality-explorer\_es.html & Math \\
29 equ & equality-explorer-basics\_1.0/2 equality-explorer-basics\_es.html & Math \\
30 exp & equality-explorer-two-variables/1.0.12/equality-explorer-two-variables\_es.html & Math \\
31 far & expression-exchange/1.14/expression-exchange\_es.html & Math \\
32 for & faradays-law/1.12/s/faradays-law\_es.html & Physics \\
33 tra & forces-and-motion-basics/2.3.16/forces-and-motion-basics\_es.html & Physics \\
34 tra & fractions-equality\_1.1.1/fractions-equality\_es.html & Math \\
35 tra & fraction-matter/1.2/fraction-matcher\_es.html & Math \\
36 tra & fractions-intro/1.0.12/fractions-intro\_es.html & Math \\
37 tri & fractions-mixed-numbers/1.0.12/fractions-mixed-numbers\_es.html & Math \\
38 fun & friction/1.5.10/friction\_es.html & Physics \\
39 fun & function-builder/1.0.23/function-builder\_es.html & Math \\
40 gas & function-builder-basics\_1.0/14/function-builder-basics\_es.html & Math \\
41 gas & gases-intro/1.0/5/gas-intro\_es.html & Earth science \\
42 gen & gas-properties/1.0/4/gas-properties\_es.html & Earth science \\
43 gra & gene-expression-essentially/1.0.16/gene-expression-essentially\_es.html & Biology \\
44 gra & graphing-lines/1.3.10/graphing-lines\_es.html & Math \\
45 gra & graphing-quadrates/1.15/graphing-quadrates\_es.html & Math \\
46 gra & graphing-slope-interpret/1.19/graphing-slope-intercept\_es.html & Math \\
47 gra & gravity-and-orbits/1.1.15/gravity-and-orbits\_es.html & Physics \\
48 gra & gravity-force-lab/2.2.0/gravity-force-lab\_es.html & Physics \\
49 hoo & gravity-force-lab-basics/1.0/gravity-force-lab-basics\_es.html & Physics \\
50 iso & hookes-law/1.0.23/hookes-law\_es.html & Chemistry \\
51 joh & isotopes-and-atomic-mass/1.19/isotopes-and-atomic-mass\_es.html & Chemistry \\
52 lea & john-travoltage/1.5.12/john-travoltage\_es.html & Physics \\
53 mak & least-squares-regression/1.1.20/least-squares-regression\_es.html & Math \\
54 mas & make-a-ten/1.0.16/make-a-ten.es.html & Math \\
55 mas & masses-and-spring\_es.10/masses-and-springs\_es.html & Math \\
56 mol & masses-and-spring\_bases-1.0/masses-and-springs-basics\_es.html & Physics \\
57 mol & molarity/1.5.1/molarity\_es.html & Chemistry \\
58 mol & molecules-and-light/1.4.14/molecules-and-light\_es.html & Earth science \\
59 mol & molecule-shapes/1.28/molecule-shapes\_es.html & Chemistry \\
60 mol & molecule-shapes-basics/1.2/smolecule-shapes-basics\_es.html & Chemistry \\
61 neu & neuro/1.1.18/neuron\_es.html & Biology \\
62 ohm & ohms-law/1.4.7/ohms-law\_es.html & Math \\
63 pen & pendulum-lab/1.0/15/pendulum-lab\_es.html & Math \\
64 ph & ph-scale/1.3/4 ph-scale\_es.html & Earth science \\
65 ph & ph-scale-basics/1.3/4 ph-scale-basics\_es.html & Biology \\
66 pi & phiko-probability\_1.11/pinko-probability\_es.html & Math \\
67 pro & projectile-motion/1.05/projectile-motion\_es.html & Math \\
68 pro & proportion-playground/1.0.15/proportion-playground\_es.html & Math \\
69 rea & reactants-products-and-leftovers/1.2.11/reactants-products-and-leftovers\_es.html & Chemistry \\
70 res & resistance-in-a-wire\_es.html & Math \\
71 rut & rutherford-scattering/1.9/trutherford-scattering\_es.html & Chemistry \\
72 sta & states-of-matter-basics\_1.18/states-of-matter-basics\_es.html & Physics \\
73 sta & states-of-matter/1.1.10/states-of-matter\_es.html & Physics \\ \hline \hline \end{tabular}
\end{table}