1. Introduction

2. Observations and Data Reduction  
2.1. Target  
2.2. Observations  
2.3. Data Reduction  
2.4. Ancillary Data  
2.5. Final Data Products  

3. Physical Parameter Estimation  
3.1. Line Ratio  
3.2. Spectral Stacking  
3.3. Total Infrared Surface Brightness  
3.4. Molecular Gas Mass Surface Density  

4. Results  
4.1. CLAWS Line Emission  
4.2. CO Emission Line Ratios  
4.3. Line Ratios and the Galactic Environment  
4.4. Spiral Arm and Interarm Variations  

5. Discussion  
5.1. Comparing R21 with Previous Studies  
5.2. Interpreting Variations of Other Line Ratios  
5.3. Comparing CO Line Ratios to Simulations  
5.4. Implications from CO Isotopologue Line Ratio Trends  
5.5. CO Spectral Line Energy Distribution  
5.6. Molecular Lines and Systematic Density Variation  

6. Conclusion