If vector $a⃗=20î$ and vector $b⃗=50ĵ$ calculate $a ⃗-b ⃗$ using $î$ and $ĵ$ notation.

• A. $-30ĵ$
• B. $20î-50ĵ$
• C. $30î̂$
• D. $20î+50ĵ$