Six Democrats are competing in Tuesday’ primary for two open seats on the Arlington County Board. It is the first time in 30 years that Arlington has had two open board seats. The two top vote-getters will be on the general election ballot in November and, in Democratic-leaning Arlington, will be heavily favored. Audrey Clement, a perennial Green Party candidate, and Michael McMenamin, who has twice run for the board as a Republican, will be on the November ballot as independents.