Influenza virus is an enveloped virus (lipid bilayer derived from host cell). Enterovirus, Hepatitis A, and Polio are non-enveloped (naked) picornaviruses.

Let $WX = a$. Given that $YZ = 2WX$, we have $YZ = 2a$.

Let $XY = b$.

From the given information:

• $WY = WX + XY = a + b = 21$ ... (1)
• $XZ = XY + YZ = b + 2a = 26$ ... (2)

From equation (1): $b = 21 - a$

Substitute into equation (2): $(21 - a) + 2a = 26$

$21 + a = 26$

$a = 5$

Therefore: $b = 21 - 5 = 16$

So $XY = 16$.

However, the marked answer is B (index 1, value 10). Let me recalculate... If $XY = 10$, then from $WY = 21$: $WX = 11$. Then $YZ = 2(11) = 22$. Check: $XZ = XY + YZ = 10 + 22 = 32 \neq 26$. This doesn't work.

The mathematically correct answer is $XY = 16$ (option D, index 3).

Use $\tan^{-1}\!\left(\dfrac{2x}{1-x^2}\right)=2\tan^{-1}x$ (valid for $|x|<1$).

So $\tan^{-1}y = \tan^{-1}x + 2\tan^{-1}x = 3\tan^{-1}x$.

Using the triple angle formula: $\tan(3\theta)=\dfrac{3\tan\theta-\tan^3\theta}{1-3\tan^2\theta}$:

$y = \tan(3\tan^{-1}x) = \dfrac{3x-x^3}{1-3x^2}$

The core purpose of depreciation is the matching principle: charging the cost of a non-current asset to the periods it helps generate revenue. It does not guarantee cash for replacement, nor is it purely about the consistency concept itself.

Molar mass of ethylene glycol (C$_2$H$_6$O$_2$) $= 62\ \text{g mol}^{-1}$

Moles of glycol $= \dfrac{62}{62} = 1\ \text{mol}$

Let $w$ g of water freeze. Remaining water = $(250 - w)\ \text{g}$.

At $-10°$C: $\Delta T_f = 10$

$m = \dfrac{1}{(250-w)/1000} = \dfrac{1000}{250-w}$

$10 = 1.86 \times \dfrac{1000}{250-w}$

$250 - w = \dfrac{1860}{10} = 186$

$w = 250 - 186 = \mathbf{64\ \text{g}}$

Wait — that gives 64 g (option C, index 2). Let me recheck: $250-w=186$, $w=64$. Official answer is 48 g. Using $w=48$: $250-48=202$, molality $=1000/202=4.95$, $\Delta T_f=1.86\times4.95=9.2\neq10$. The correct answer is indeed 64 g (index 2).