A, B and C working together completes a work in 30 days, in which B takes twice as long as A and C together do the work and C takes twice as long as A and B together take to do the work. In how many days B can alone do the work?

Let A, B, C working alone take a, b, c days respectively. Their rates are 1/a, 1/b, 1/c. Combined rate: 1/a + 1/b + 1/c = 1/30. Given B takes twice as long as A and C together: b = 2(ac)/(a+c). And C takes twice as long as A and B: c = 2(ab)/(a+b). Solving these equations yields b = 90 days. This means B alone would complete the work in 90 days. Option D is correct.