Let $ABCDA'B'C'D'$ be a rectangular prism with $|AB|=2|BC|$. $E$ is a point on the edge $[BB']$ satisfying $|EB'|=6|EB|$. Let $F$ and $F'$ be the feet of the perpendiculars from $E$ at $\triangle AEC$ and $\triangle A'EC'$, respectively. If $m(\widehat{FEF'})=60^{\circ}$, then $|BC|/|BE| = ? $

$ \textbf{(A)}\ \sqrt\frac53 \qquad \textbf{(B)}\ \sqrt\frac{15}2 \qquad \textbf{(C)}\ \frac32\sqrt{15} \qquad \textbf{(D)}\ 5\sqrt\frac53 \qquad \textbf{(E)}\ \text{None}$
1. **Given Information and Setup:**
   - We have a rectangular prism \( ABCDA'B'C'D' \) with \( |AB| = 2|BC| \).
   - Point \( E \) is on the edge \( [BB'] \) such that \( |EB'| = 6|EB| \).
   - \( F \) and \( F' \) are the feet of the perpendiculars from \( E \) to \( \triangle AEC \) and \( \triangle A'EC' \), respectively.
   - The angle \( \angle FEF' = 60^\circ \).

2. **Relationship Between \( AB \) and \( BC \):**
   - Let \( |BC| = x \). Then \( |AB| = 2x \).

3. **Position of Point \( E \):**
   - Since \( |EB'| = 6|EB| \), let \( |EB| = y \). Then \( |EB'| = 6y \).
   - The total length \( |BB'| = |EB| + |EB'| = y + 6y = 7y \).

4. **Perpendiculars and Rectangles:**
   - Since \( F \) and \( F' \) are the feet of the perpendiculars from \( E \) to \( \triangle AEC \) and \( \triangle A'EC' \), respectively, \( BF \perp AC \) and \( B'F' \perp A'C' \).
   - Thus, \( BB'FF' \) forms a rectangle.

5. **Right Triangle and Altitude:**
   - In \( \triangle ABC \), which is a right triangle, \( BF \) is the altitude to the hypotenuse \( AC \).
   - Using the property of right triangles, the altitude to the hypotenuse is given by:
     \[
     BF = \frac{|AB| \cdot |BC|}{|AC|}
     \]
   - Since \( |AB| = 2x \) and \( |BC| = x \), we need to find \( |AC| \):
     \[
     |AC| = \sqrt{|AB|^2 + |BC|^2} = \sqrt{(2x)^2 + x^2} = \sqrt{4x^2 + x^2} = \sqrt{5x^2} = x\sqrt{5}
     \]
   - Therefore,
     \[
     BF = \frac{2x \cdot x}{x\sqrt{5}} = \frac{2x^2}{x\sqrt{5}} = \frac{2x}{\sqrt{5}} = \frac{2x\sqrt{5}}{5}
     \]

6. **Finding \( BC/BE \):**
   - The problem asks for \( \frac{|BC|}{|BE|} \).
   - Given \( \angle FEF' = 60^\circ \), we use the tangent relationship:
     \[
     \tan \angle FEB = a \quad \text{and} \quad \tan \angle F'EB' = \frac{a}{6}
     \]
   - Using the tangent addition formula:
     \[
     \tan 120^\circ = \tan (\angle F'EB' + \angle FEB) = \frac{a + \frac{a}{6}}{1 - a \cdot \frac{a}{6}} = \frac{\frac{7a}{6}}{1 - \frac{a^2}{6}}
     \]
   - Since \( \tan 120^\circ = -\sqrt{3} \):
     \[
     -\sqrt{3} = \frac{\frac{7a}{6}}{1 - \frac{a^2}{6}}
     \]
     \[
     -\sqrt{3} \left(1 - \frac{a^2}{6}\right) = \frac{7a}{6}
     \]
     \[
     -\sqrt{3} + \frac{a^2 \sqrt{3}}{6} = \frac{7a}{6}
     \]
     \[
     -6\sqrt{3} + a^2 \sqrt{3} = 7a
     \]
     \[
     a^2 \sqrt{3} - 7a - 6\sqrt{3} = 0
     \]
     \[
     a = 3\sqrt{3}
     \]

7. **Final Calculation:**
   - Using \( a = 3\sqrt{3} \):
     \[
     \frac{|BC|}{|BE|} = \frac{\sqrt{5} \cdot \tan \angle FEB}{2} = \frac{\sqrt{5} \cdot 3\sqrt{3}}{2} = \frac{3\sqrt{15}}{2}
     \]

The final answer is \(\boxed{\frac{3}{2}\sqrt{15}}\)