answer:To solve this problem, let's denote the cost of the T-shirt as x yuan. The store makes a profit of 20% on this cost, which can be represented as 0.20x. The T-shirt is sold at a 20% discount on the marked price of 240 yuan, so the selling price becomes 0.8 times 240 yuan. The equation representing the relationship between the cost, the profit, and the selling price is: [x + 0.20x = 0.8 times 240] Combining like terms on the left side gives us: [1.20x = 0.8 times 240] Simplifying the right side: [1.20x = 192] To find x, we divide both sides by 1.20: [x = frac{192}{1.20}] [x = 160] Therefore, the cost of the T-shirt is 160 yuan. So, the correct answer is option boxed{B: 160 text{ yuan}}.

answer:First, we have overrightarrow {ab} = frac { overrightarrow {a}cdot overrightarrow {b}}{ overrightarrow {b}^{2}} = frac {| overrightarrow {a}|cosθ}{| overrightarrow {b}|} ∈ {frac {n}{3}|n∈Z} and overrightarrow {ba} = frac { overrightarrow {b}cdot overrightarrow {a}}{ overrightarrow {a}^{2}} = frac {| overrightarrow {b}|cosθ}{| overrightarrow {a}|} ∈ {frac {n}{3}|n∈Z}. Since |overrightarrow {a}|≥|overrightarrow {b}|>0, we can set m,t∈Z such that overrightarrow {ab} = frac {m}{3} and overrightarrow {ba} = frac {t}{3}. This gives us cos^2θ = frac {mt}{9} ∈ ( frac {3}{4} , 1). By assigning values to m and t, we find frac {27}{4} < mt < 9, which leads to mt = 7 or mt = 8. Taking m = 4, 7, 8, we find the possible values for overrightarrow {ab} are frac {4}{3}, frac {7}{3}, and frac {8}{3}. Therefore, the correct answer is: boxed{D. overrightarrow {ab} = frac {4}{3}, frac {7}{3}, frac {8}{3}}. This problem tests the understanding of vector dot products, set properties, and reasoning and computational abilities. It is considered a moderately difficult problem.

answer:The general term in the expansion of (1+kx^2)^6 according to the binomial theorem is T_{r+1} = C_6^r(kx^2)^r = C_6^r k^r x^{2r}. Therefore, the coefficient of x^8 is C_6^4 k^4 = 15k^4. This means 15k^4 = 240, which simplifies to k^4 = 16, leading to k = pm2. Hence, the answer is boxed{pm2}.

answer:**Analysis** This question tests the conversion between exponentiation and logarithms. The result can be obtained based on the concept of logarithms. It is a basic question. **Solution** Given (a^{ frac{1}{3}}=b), According to the concept of logarithms, we have (log_{a}b= frac{1}{3} ), where (a > 0) and (a neq 1). Therefore, the correct answer is boxed{text{B}}.