The number of real roots of the equation sin⁴ θ – 2 sin² θ – 1 = 0 in the interval (0, 2π) is 

(a) 0 

(b) 1 

(c) 2 

(d) 4

Two taps A and B can fill an overhead tank in 10 hours and 15 hours respectively. Both the taps are opened for 4 hours and then B is turned off. How much time will A take to fill the remaining tank?

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श्रम के प्रकार बताइए।