In a certain code language, 'MUTUAL' is written as 'OXVXCO' and 'REASON' is written as 'THCVQQ'. How will 'SERIES' be written in that language?

Let's investigate the pattern of the letters:For MUTUAL $\to$ OXVXCO:$\text{M}\stackrel{+2}{\to }\text{O}$$\text{U}\stackrel{+3}{\to }\text{X}$$\text{T}\stackrel{+4}{\to }\text{X}$$\text{U}\stackrel{+3}{\to }\text{X}$$\text{A}\stackrel{+2}{\to }\text{C}$$\text{L}\stackrel{+3}{\to }\text{O}$ Wait, let's look at a simpler or more uniform pattern:$\text{M}(13)+2=\text{O}(15)$$\text{U}(21)+3=\text{X}(24)$$\text{T}(20)+4=\text{X}(24)$$\text{U}(21)+4=\text{Y}$? But it is $\text{B}$ or $\text{C}$? Let's check position indices:$\text{MUTUAL}$:$\text{M}\to \text{O}(+2)$$\text{U}\to \text{X}(+3)$$\text{T}\to \text{V}(+2)$? The question text says OXVXCO.Let's check letter-by-letter index changes for MUTUAL to OXVXCO:$\text{M}(13)+2=15(\text{O})$$\text{U}(21)+3=24(\text{X})$$\text{T}(20)+2=22(\text{V})$$\text{U}(21)+3=24(\text{X})$$\text{A}(1)+2=3(\text{C})$$\text{L}(12)+3=15(\text{O})$ So the pattern is alternating additions: $+2,+3,+2,+3,+2,+3$.Let's double-check with REASON $\to$ THCVQQ:$\text{R}(18)+2=20(\text{T})$$\text{E}(5)+3=8(\text{H})$$\text{A}(1)+2=3(\text{C})$$\text{S}(19)+3=22(\text{V})$$\text{O}(15)+2=17(\text{Q})$$\text{N}(14)+3=17(\text{Q})$ The pattern $+2,+3,+2,+3,+2,+3$ is confirmed.Now apply this pattern to SERIES:$\text{S}(19)+2=21(\text{U})$$\text{E}(5)+3=8(\text{H})$$\text{R}(18)+2=20(\text{T})$$\text{I}(9)+3=12(\text{L})$$\text{E}(5)+2=7(\text{G})$$\text{S}(19)+3=22(\text{V})$ The encoded word is UHTLGV.